The American Cyclopædia (1879)/Geology

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Edition of 1879. See also Geology on Wikipedia, and the disclaimer.

GEOLOGY (Gr. γῆ, the earth, and λόγος, discourse), the science which treats of the structure of the earth, and of the methods by which its materials have been arranged. Under this term are confounded two distinct branches of study, the one being that of the chemical, physical, and biological laws which have presided over the development of the globe, and the other the natural history of the earth as displayed in its physical structure, its stratigraphy, mineralogy, and palæontology. The name of geognosy, employed by some authors, may be very appropriately retained for the latter, while that of geogeny may be restricted to the first or theoretical division of geology. A knowledge of physical geography, of the distribution of land and water in past and present times, and of the laws of winds, currents, and climates, is one of the first requisites in the study of geology. Then comes the investigation of the various kinds of rocks, their arrangement and structure, their succession and relative antiquity, their chemical and mineralogical history. The investigation of the chemical agencies which have presided over the formation of the various kinds of rocks and minerals belongs to chemical geology, while the laws which have regulated their deposition, structure, and arrangement constitute dynamical geology. The student finds that organic life in past time played a part in the earth not less important than it does to-day, and the study of the organic remains found in the various rocky strata, and known as fossil plants and animals, gives rise to departments of botany and zoölogy which are sometimes called palæobotany and palæozoölogy, but are more generally included under the common term of palæontology. The changes that have taken place in the inorganic and organic world introduce in their study considerations of time and progress, and the science is found to he largely of a historical character; the geologist, as Cuvier remarked, being an antiquary of a new order. Its historical element is regarded by Lyell as so prominent that he defines geology simply as “the science which investigates the successive changes that have taken place in the organic and inorganic kingdoms of nature.” In the present article little more will be attempted than to present a general sketch of the history and progress of geological science, a reference to some principal objects of its pursuit, and the system of classifying the groups of rocks generally adopted. The history of the science as developed in Europe is minutely traced in the familiar work of Lyell, “Principles of Geology,” in which the whole subject may also be most advantageously studied.—From the earliest times the structure of the earth has been an object of interest to man, not merely on account of the useful materials he obtained from its rocky formations, but also for the curiosity awakened by the strange objects it presented to his notice. The south and west of Asia and much of the country bordering the Mediterranean were particularly favorable for directing attention to geological phenomena. Earthquakes were frequent, changing the relative positions of sea and land; volcanoes were seen in operation, adding layers of molten rock to those of sand and mud filled with the shells of the Mediterranean; the strata in the hills abounded in evidences of similar collections of vestiges of marine life far removed from access of the sea, and yet unchanged during the period of human observation and tradition; the Ganges and the Nile, pouring forth their vast sedimentary accumulations, were plainly building up the deltas at their mouths, and the broad valleys reaching far up their course were unmistakable productions of the same series of operations in remote periods. These phenomena could not escape the attention of the philosophers among the ancient Egyptians and Indian races; and their influence is perceived in the strange mixtures of correct observation and extravagant conceit which make up their cosmogonies or universal theories of the creation. In the first chapter of the ordinances of Manu alternating periods of destruction and of renovation are distinctly recognized, extending in eternal succession throughout the whole assemblage of locomotive and immovable creatures, each period comprehending a duration of many thousand ages. The Greek schools of philosophy recognized these phenomena, which were clearly enunciated by Ovid in presenting the doctrines of Pythagoras. Remarkably free from extravagant statements, they were applied to prove a system of perpetual change slowly modifying the surface of the earth. Aristotle recognized the interchanges constantly taking place between land and sea by the action of running water and of earthquakes, and remarked how little man, in the short span of his life, can perceive of operations extending through the eternity of time. Strabo distinctly applied the raising up of land, not merely of small tracts, but of continents also, by earthquake convulsions, to account for the perplexing phenomenon of beds of marine shells contained in the interior of hills far distant from the sea. Arabian philosophers of the 10th century are also cited who entertained similar views of the changes going on and their causes.—The Italian philosophers in the early part of the 16th century were the first to engage in systematic investigations concerning the true nature of fossil shells. Their abundance in the strata of the sub-Apennine range could not fail to arrest attention and excite inquiries, which were the more perplexing from the limited time allowed in popular belief to the past duration of the earth, and from the general persuasion that no great catastrophe except the Noachian deluge could have occurred to modify its surface. Various fanciful explanations were therefore adopted in the spirit of the scholastic disputations, and for three centuries argumentations were sustained with much spirit on the questions: first, whether fossil remains had ever belonged to living creatures; and secondly, admitting this, whether all the phenomena could not be explained by the deluge of Noah. Among those distinguished for the soundness of their views in the commencement of this controversy are Leonardo da Vinci, the celebrated painter, who died in 1519, and Fracastoro, whose attention was engaged by the multitude of curious petrifactions which were brought to light in 1517 in the mountains of Verona, in quarrying materials for repairing the city. He exposed the absurdities of the theories which referred the petrifactions to a certain plastic force in nature that could fashion stones into organic forms, and showed the inadequacy of the traditional deluge to bring together the marine fossils that form solid strata of the earth. About this time collections of these curiosities were made for public museums and private cabinets; they were deposited in the museum of the Vatican at Rome, and that of Canceolarius at Verona became famous for them. Descriptive catalogues of these collections were published; and as early as 1565 appeared one of the collection of J. Keutman in Gesner's work De Rerum Fossilium, Lapidum et Gemmarum Figuris. In 1580 Palissy was the first who dared assert in Paris that fossil remains of testacea and fishes had once belonged to marine animals. The truth made but slow progress in the face of established prejudices. In 1669 Steno, professor of anatomy at Padua, published his work De Solido intra Solidum naturaliter Contento, in which he proved the identity of the fossil teeth found in Tuscany with those of living sharks, and the close similarity of the fossil testacea to living species; he traced their progressive change from unaltered shells to solid petrifactions, and recognized the distinction between formations deposited by salt and by fresh water, and that some were of an earlier period than the introduction of plants and animals upon the earth. But neither he nor Scilla, the Sicilian painter, who in his Latin treatise on the fossils of Calabria, illustrated by good engravings (1670), ably maintained the organic nature of fossil shells, ventured to refer their occurrence in the strata to any other cause than the Mosaic deluge. Leibnitz, the great mathematician, in his Protogæa (1680), first proposed the theory of the earth having originally been a burning luminous mass, which since its creation has been cooling down, and as it cooled received the condensed vapors which now compose its crust. In one stage of its formation he believed it was covered with a universal ocean. From these materials Leibnitz traced two classes of primitive formations, the one by refrigeration from igneous fusion, the other by concretion from aqueous solution. The first recognition of the arrangement of the earthy materials in strata, continuous over large areas, and resembling each other in different countries, appears to have been by Dr. Lister, who sent to the royal society of London in 1683 a proposal for maps of soils or minerals. He also believed that species had in past ages become extinct. Dr. Robert Hooke near the close of the 17th century prepared a “Discourse on Earthquakes,” which contains the most philosophical views of the time respecting the nature of fossils and the effects of earthquakes in raising up the bed of the sea. William Woodward was a distinguished observer of the geological formations of Great Britain, and perceived that the lines of outcrop of the strata were parallel with the ranges of the mountains. About 1695 he formed a collection of specimens, which he systematically arranged and bequeathed to the university of Cambridge. For this he purchased the original specimens and drawings of fossil shells, teeth, and corals of Scilla. But his geological system was cramped by the attempt to make it conform to the received interpretation of the Scriptural account of the creation and deluge. The Italian geologists Vallisneri in 1721, Moro in 1740, and Generelli in 1749, advanced the most philosophical views yet presented respecting the fossiliferous strata, and sustained them by original observations made by the first two throughout Italy and among the Alps. Moro endeavored to make the production of strata correspond in time to the account of the creation of the world in six days, and hence was compelled to refer them to volcanic ejections, which by floods, he imagined, were distributed over the surface of the earth and piled up in strata with marvellous celerity. Buffon advanced views respecting the formation and modification of mountains and valleys by the action of water, in his “Natural History” (1749), a portion of which, contained in fourteen propositions, he was required by the faculty of theology in Paris to renounce. This he did in his next work, accompanying the formal abandonment of what he had written contrary to the narration of Moses with a declaration of belief of all contained in the Scripture about the creation, both as to order of time and matter of fact.—Geology did not begin to assume the rank of an important science until its application to the practical purposes of mining was first pointed out in the last quarter of the 18th century by Werner, professor of mineralogy in the school of mines at Freiberg in Saxony. This distinguished man attracted pupils from distant countries, and sent them forth enthusiastic geologists and advocates of the views he had conceived from his imperfect observation of the geology of a small portion of Germany. He taught the systematic order of arrangement of the strata, adopting nearly the same divisions that had been proposed fifty years previously by Lehmann, a German miner. He explained their production as the result of precipitation from a common menstruum or “chaotic fluid,” which he supposed had once covered the whole surface of the earth. As expounded by Jameson in 1808, the first precipitates from this ocean were chemical, and produced the crystalline rocks which lie at the base of all the others, and which he designated as the primitive class. They included the granitic rocks and those called crystalline schists, such as gneiss, mica slate, clay slate, serpentine, &c. The second class comprised the rocks he calls transition, certain limestones, flinty slate, gypsum, graywacke, and trap, most of which are probably now included in the palæozoic formations. They were supposed to have been formed during the transition of the earth from its chaotic to its habitable state, and to have been partly chemical and partly mechanical in their origin, and due to the action of the waves and currents. The third class contained the rocks denominated Flötz, because as observed in Germany they were disposed in horizontal or flat strata. In this were the coal formation, various sandstones, the chalk, rock salt, gypsums, various limestones, and certain traps. They were supposed to have been formed while animals and vegetables existed in numbers, and to have been partly chemical and partly mechanical in their origin. The fourth class contained the alluvial rocks, those produced on the land, as peat, sand and gravel, loam, bog iron ore, calc tuff, &c., being understood to comprise all above the chalk excepting the volcanic. The fifth class comprised the volcanic rocks, the pseudo-volcanic, and the true volcanic; the former being the supposed products of the combustion of coal and sulphurous matters, the latter of real volcanoes. These formations were supposed to be systematically arranged; the later formed either entirely covering the older, or, when these form a central mountain mass, encircling this, so that the “outgoings” of the strata (meaning their upper edges or lines of outcrop) form circles; those of the later formed groups being successively larger. The basin and trough-shaped deposits were also recognized, in which the outgoings of the newer strata became successively smaller. The strata, it was understood, were subject to local disturbances from portions sinking into subterranean cavities, and members might be wanting in some localities, but whenever present must be found in their proper position in relation to the others. Basalt, which in Saxony and Hesse was seen capping the hills of stratified rocks, he inferred must be of the same series of precipitated formations, although many other geologists of Werner's time had fully established the analogy between this rock and modern lavas. The observations of Desmarest, especially in the district of extinct volcanoes in Auvergne, made in 1768, are referred to by Lyell as most clearly tracing the origin of the basalts to the craters of the volcanoes. A new controversy now arose, which for many years was waged with animosity and bitterness unprecedented in disputes of this class. Geologists throughout Europe were divided into the two classes of Neptunists, who advocated the production of the rocks by aqueous deposition alone, and Vulcanists, who attributed the origin of many of them to the action of fire. They were also called, from the names of their respective leaders, Wernerians and Huttonians. Dr. Hutton of Edinburgh had studied geology for himself in different parts of Scotland and England, and formed his own conclusions, which he ably sustained. He was the first to announce that geology had no concern with questions as to the origin of things, but that the true field of its investigations was limited to the observation of phenomena and the application of natural agencies to explain former changes. His friend Sir James Hall showed by actual experiment that the prismatic structure of basalt might result in cooling from a state of igneous fusion; and Hutton himself found in the Grampian hills the granite branching out in veins, which extended from the main body through the contiguous micaceous slates and limestone, thus indicating its having been in a fused state at a time subsequent to the production of Werner's primitive rocks. This discovery soon led to questioning the existence of any primitive class of rocks the origin of which lay beyond the reach of the present order of things; and the announcement made by Hutton, “In the economy of the world I can find no traces of a beginning, no prospect of an end,” may well have startled men of science and shocked the religious public in the sensitive condition to which it had been brought by the infidel doctrines promulgated in the latter part of the last century, especially by men of letters in France. The Vulcanists came to be classed with the enemies of Scripture, the true object of investigation was lost sight of, and the controversy was continued with such animosity that the party names at last became terms of reproach, and many geologists avoided being involved in it. Workers in the field, however, were collecting new and valuable data that were to give to the science a more exact character. William Smith, a civil engineer, prepared in 1793 a tabular view of the strata near Bath, tracing out their continuity over extensive areas, and recognizing them by the fossils they contained. This method of identification and of arranging strata in their true positions he taught himself, and was the first to promulgate in England. With extraordinary perseverance he continued to prosecute his work alone, travelling on foot over all England, freely communicating his observations, and in 1815 he completed a geological map of the whole country. In France the importance of fossils as characteristic of formations was also beginning to be appreciated. Lamarck and Defrance earnestly engaged in the study of fossil shells, and the former in 1802 reconstructed the system of conchology to introduce into it the new species collected by the latter in the strata underlying the city of Paris. Six years previous to this Cuvier had established the different specific character of fossil and living elephants, which opened to him, as he said, views entirely new respecting the theory of the earth, and determined him to devote himself to the researches which occupied the remainder of his life. In 1807 the geological society of London was established, with the professed object of encouraging the collection of data, multiplying and recording observations, with no reference to any “theories of the earth.” Its active members completed the classification and description of the secondary formations of Great Britain, so well commenced by William Smith; while at the same time the tertiary formations were thoroughly investigated by Cuvier, Brongniart, and others in Paris. Thus each country contributed to the advancement of geological science in the department connected with its most prominent formations: Germany in that of the lower stratified and crystalline rocks, and especially in the mineralogical structure of these, while in Scotland the character of the granitic rocks had been more particularly elucidated, in England that of the secondary strata and their order of arrangement, and in France the tertiary. The great principles gradually developed by these observations were: that the materials of the stratified rocks were sedimentary deposits that had slowly accumulated in the beds of ancient seas and lakes; that each stratum represented a certain period during which its materials were gathered, and that this period was characterized by its peculiar group of organized beings, the vestiges of which were buried and remained with it as records of the condition of this portion of the earth during this time. The piles of strata of various kinds indicated changes in the character of the deposits introduced, sandstones formed from sand, alternating with shales formed from muddy and clayey deposits, and with calcareous strata, whose origin may have been in marl beds or the remains of calcareous organisms. The long succession of these strata, in connection with the evidences of their slow accumulation, observed in the undisturbed condition of the fossil remains which they contained, bore witness to long periods occupied in the production of a single group of strata constituting but a minor division of one of the formations. The lapse of long periods was also indicated by the fossils found in beds of older date becoming constantly more and more unlike existing species. The same localities, too, presented in their successive beds some that were filled with marine vestiges alone, corallines and sea shells, in layers of such thickness that ages must have passed while they were quietly accumulating; and above or below these were found other strata indicating that the surface at another period was covered with fresh water, the organic remains which they contained being only of the character of those belonging to ponds and rivers; and yet again these localities became dry land, and were covered with the forests of tropical climes, and peopled with numerous strange species of animals, whose nearest living analogues are met with only in hot countries. Such changes as these also plainly marked slowly progressing revolutions, the period of which no one could compute by years. It was apparent that the sediments had collected as beds of sand and clay now collect in seas and lakes, and especially about the mouths of large rivers; but it was only in such as were evidently the product of the streams of the present day that the organic vestiges were recognized as belonging entirely to familiar species. In these alone were discovered any relics of man or any indications of his existence; and here they were not wanting, for in the calcareous strata in process of formation and filled with recent species of shells human remains have been found. But with the first step backward the bones of extinct gigantic mammalia introduce us to strange groups of animals, and no satisfactory evidence is afforded, either in the strata or in tradition, that man was their contemporary. Thus in the closest connection, geologically speaking, are we presented with the most striking examples of other great principles developed by geological research, viz., the extinction of old and the introduction of new species.—In consequence of the system of observation and close investigation now established, geology lost its highly speculative character, and rapid progress continued to be made in acquiring correct information of the arrangement of the strata of different countries. While the defects of Werner's classification were exposed, the general plan of it was seen to be founded in nature, and attention was directed to collecting everywhere the materials for filling out the vertical column of the rocks, as well as mapping them throughout their horizontal range. In every country some formations could be recognized, from which as a base a local classification might proceed to contiguous groups, and thus at last the whole be included in one system of classification. So the work of descriptive geology has ever since been going on, new discoveries continually adding to its completeness and helping to the compilation of a perfect system, which in this case should present a full chart of the rocks from the lowest or oldest to the uppermost or newest. Strata lying in juxtaposition in one region, when identified in another, are found to be separated by the interpolation of a new series; and again, in tracing out over broad areas a group of sedimentary strata, they are found gradually to assume new features, and even to undergo an entire change of chemical composition. The deposits over different parts of the ocean's bed are found to be here sands and gravels brought by currents, and there soft calcareous muds, the remains of minute animal organisms accumulated in still waters. The organic remains as well as the mineral character of these contemporary deposits present wide differences. From the mode of their formation it is evident that all stratified formations must be of limited area, and must thin away at their edges, presenting the shape of lenticular sheets lapping upon each other.—In 1819 the geological society of London, through the labors of Mr. Greenough and his friends, published a map of England which was a great improvement upon that of Smith. About the same time Leopold von Buch prepared a similar map of a large part of Germany. A geological survey of France was ordered in 1822 by the French government, by which a complete geological map of France was finally constructed in 1841. M. Bronchant de Villiers, professor in the school of mines, was appointed to take charge of the work, and with him were associated Élie de Beaumont and Dufrénoy. The attention of these geologists was first given to an examination of the strata above the coal formation of England, where they had been most carefully studied and particularly described by Conybeare and Phillips in their treatise on “The Geology of England and Wales” (1821). The secondary strata of Germany also were familiar to geologists; and both countries consequently furnished important points of reference for the arrangement of the groups of France. The chalk formation of Paris, the upper member of the secondary, served as the starting point, and proceeding from this they examined in detail the lower strata as they appeared successively emerging from beneath it, and identified them, as they could, with the corresponding groups of other countries. Such is the method ever since pursued, by which our knowledge of the strata which make up the outer crust of the earth has been systematically extended. The importance of the organic remains found in the rocks has been more and more appreciated, and the shells constituting the chief portion of these have been most thoroughly studied; for while the different formations or groups of strata may contain numerous similar beds of limestone, sandstone, slates, and shales, not to be distinguished by their mineral characters, and which frequently cannot be traced to their meeting with other known formations by which their place or relative positions may be determined, the fossils show no such indiscriminate distribution. Each period was characterized by its peculiar group of animated beings, and if their arrangement is understood it follows that the position of any stratum in which the fossils are recognized must also be determined. A single species may in some cases be peculiar to one member of a geological formation, and serve wherever the fossil is found to identify the rock; but usually in different countries their identification by fossils is dependent upon characteristic genera and the order of succession of their principal groups. This branch of the subject will be more particularly treated in the article Palæontology.—In the latter part of the last and early part of the present century papers upon geological subjects occasionally appeared in the transactions of the American philosophical society of Philadelphia, the transactions of the American academy, and in other scientific journals. The character of these papers is almost exclusively descriptive. There is, however, a theory of the earth proposed by Franklin in the “Philosophical Transactions” of 1793; and in vol. vi. appeared the memorable essay of William Maclure, read Jan. 20, 1809, entitled “Observations on the Geology of the United States, explanatory of a Geological Map.” The author of this paper had undertaken a more arduous and gigantic work even than that which was occupying William Smith of England; it was no less than a geological survey of the United States alone and at his sole expense—a work which entitled him to the appellation he has received of the father of American geology. In this pursuit he crossed the Alleghanies fifty times, visited almost every state and territory in the Union, and for years continued his labors mostly among those who could have no appreciation of his objects. He had visited nearly all the mining districts of Europe, and thus was well qualified, for one of that period, to recognize the corresponding formations of the two continents. He traced out the great groups of strata then designated as the transition, secondary, and alluvial, in their range from the St. Lawrence to the gulf of Mexico. The tertiary, however, he did not recognize, owing to the absence of the chalk formation, the upper member of the secondary, which in Europe, being largely developed and most conspicuous, marks the strata of more recent origin lying above it as tertiary. He continued his explorations after this report, and in May, 1817, presented another to the philosophical society, accompanied by a colored map and sections. His observations were also extended in 1816 and 1817 to the Antilles, and a paper upon the geology of these islands was published in the first volume of the “Journal of the Academy of Natural Sciences.” Prof. Silliman of New Haven, educated to the profession of the law, was induced by President Dwight of Yale college to qualify himself for the departments of natural science, particularly chemistry; and with this view he spent some time previous to 1806 in England and Scotland. In Edinburgh he became familiar with the discussions of the Wernerians and Huttonians in that transition period, as he styles it, between the epoch of geological hypothesis and dreams and the era of strict philosophical induction in which the geologists of the present day are trained. The interest excited by this controversy could not fail to direct his tastes toward the new science, and he returned to become its zealous promoter, for half a century or more aiding to elucidate the geology of his country, inspiring the enthusiasm of others, and furnishing in the “American Journal of Science” an organ for the diffusion of scientific knowledge. At that period (1804-'5), he says, geology was less known in the United States than mineralogy. Most of the rocks were without a name, except so far as they were quarried for economical purposes, and classification of the strata was quite unknown. Dr. Archibald Bruce of New York commenced in 1810 the publication of a journal devoted principally to mineralogy and geology, the earliest purely scientific journal supported by original American communications. It was well received at home and abroad, but appeared only at wide intervals, and ended with the fourth number. The mineralogical collections at the principal colleges, and others belonging to scientific men mostly in New York, promoted inquiry and observation concerning the geological relations of the minerals and their distribution. The admirable treatise on mineralogy by Prof. Parker Cleaveland, published in 1816, fostered while it gratified this spirit of inquiry. In 1818 the brothers Prof. J. F. Dana and Dr. Samuel L. Dana published a detailed report on the mineralogy and geology of the vicinity of Boston. In the same year was first published the “American Journal of Science,” which has continued ever since to be the chief periodical American recorder of the progress of the sciences. The next year the American geological society held its first meeting at New Haven, where it continued to meet annually for several years. The importance of geological explorations, with the view of thereby ascertaining the agricultural and mineral capacities of large districts, was beginning to be appreciated by communities and public bodies. In 1820 a geological survey of the county of Albany, N. Y., was made under the direction of the agricultural society of the county by Prof. Amos Eaton and Dr. T. E. Beck. Two years afterward Rensselaer and Saratoga counties were also thus explored. Prof. Eaton was also engaged by Gen. Stephen Van Rensselaer to make at his expense a geological survey of the country adjacent to the Erie canal. The result of this was published in 1824 in a report of 160 pp. 8vo, with a profile section of the rock formations from the Atlantic ocean through Massachusetts and New York to Lake Erie, the Rev. Edward Hitchcock furnishing many of the details through Massachusetts. The first geological survey made by state authority was that of North Carolina in 1824 and 1825, by Denison Olmsted. Since that time there have been various surveys by the different states or by the federal government, of which we shall notice the most important historically. Beginning at the northeast, early surveys were made of Maine, New Hampshire, and Rhode Island, by Dr. C. T. Jackson, in 1836-'41; of Massachusetts, by Edward Hitchcock, in 1830-'40; of Connecticut, by J. G. Percival and C. U. Shepard, in 1836, and of Vermont in 1845-'6, a work which was continued by Edward Hitchcock and his son, C. H. Hitchcock, in 1858-'60, the latter of whom is now (1874) engaged in a resurvey of New Hampshire. In 1836 was commenced the survey by H. D. Rogers and his assistants of the state of Pennsylvania, which was not completed till 1855. The survey of New York in 1836-'42, by Vanuxem, Emmons, Mather, and Hall, may be said to have opened a new era in American geology by giving a complete and systematic classification of the palæozoic rocks within its borders, which has served as a basis for all subsequent work to the east of the Rocky mountains. The description of the organic remains of the state by Prof. James Hall is still incomplete, but five large quarto volumes have been published. The surveys of Michigan in 1837-'46 by Houghton, and of the Lake Superior region in 1847-'9 by Jackson, and subsequently by J. D. Whitney and J. W. Foster, served to extend our knowledge of the palæozoic rocks to the westward. From that time to the present systematic surveys of the various states of the great Mississippi valley have been or still are in progress, and have already given us a pretty accurate knowledge of the geology of the whole of this vast region. The history of this work is too long for the present occasion, and it may seem invidious to mention names among workers in this great field; but a prominent place should be given, in addition to those just mentioned, to D. D. Owen, B. F. Shumard, Swallow, J. T. Hodge, Worthen, Newberry, Safford, E. W. Hilgard, Cox, and Tuomey. Nor should the important labors of Oscar Lieber in South Carolina and of Emmons in North Carolina be forgotten, nor the elaborate survey of Virginia by William B. Rogers, of which only partial reports have been published. The geology of the western portion of our continent presents characters widely different from that already noticed, and is now attracting great attention. Much important information was gathered by the labors of W. P. Blake and J. S. Newberry in the course of the great railroad surveys undertaken by the national government; and the geological work has been continued in the important survey of the 40th parallel under Clarence King, and that of the Rocky mountain region by J. V. Hayden. These labors are still in progress, as is also a geological survey of California under J. D. Whitney, and the great geological features of this region are being rapidly made known. Much progress has also been made in the study of the geology of British North America. A geological survey of Canada, embracing the present provinces of Ontario and Quebec, was begun in 1842 under Sir W. E. Logan, with whom were associated for many years Mr. Alexander Murray and Dr. T. Sterry Hunt. In 1870 Mr. A. R. C. Selwyn succeeded Logan in the present Dominion of Canada, including the British territory west to the Pacific, the field of the survey being thus greatly extended. The provinces of Nova Scotia and New Brunswick were early examined by Gesner, since which time Matthew Bailey, Hartt, Hind, Hunt, and Dawson have done much to develop their geology. The last named has especially studied the carboniferous rocks of that region. A survey of Newfoundland is in progress under Alexander Murray. The labors of the late Sir John Richardson, Hector, Hind, and others, have done much to elucidate the structure of the great region north of Canada, until lately known as the Hudson Bay territory.—With this brief sketch of the progress of geological research in North America, we may now proceed to discuss the general principles of geological classification, and to illustrate them by especial reference to American geology. The great groups introduced by Werner remain essentially unchanged, but many alterations in nomenclature and various subdivisions and reclassifications have since been adopted, some of which require notice. Besides the great distinction between crystalline and uncrystalline rocks is that of stratified and unstratified rocks, having reference not to their intimate structure, but to their geognostical relations. The stratified rocks include all those which appear to be arranged in beds or strata, whether crystalline or not; and the unstratified, those which, like granites, traps, basalts, and volcanic lavas, occur in masses which are destitute of such arrangement, and appear to have been forced into their present position while in a more or less softened or molten condition. These are often spoken of as eruptive, irruptive, or intrusive rocks. They are with a few exceptions crystalline, and in certain cases are not readily distinguished from those crystalline stratified rocks in which the bedding is ill defined, either from having been obscure from the first or else obliterated by subsequent crystallization. There are strong reasons for believing that the stratified crystalline rocks, by a process of softening and subsequent displacement or eruption, gave rise to the unstratified rocks with which they are often mineralogically identical; and hence the names of indigenous and exotic crystallines have been proposed by Dr. Hunt to designate respectively the stratified and the eruptive rocks. A third class of crystalline rocks is also to be distinguished, viz.: those which occur as veinstones in the fissures of other rocks, and have probably been deposited from watery solutions. Such are the quartz and spars which form the gangue of many metallic ores, and a large part of the so-called granite veins. The rocks of this third class, from their mode of formation, are designated by Dr. Hunt as endogenous crystallines. It is in some cases impossible to determine from its mineralogical characters to which of these three classes a given crystalline rock belongs. The unstratified crystalline or eruptive rocks include the modern volcanic lavas, which are evidently the products of igneous fusion, and the whole class is therefore sometimes designated as igneous rocks. It is supposed however that many of these rocks, as for example the exotic granites, have never been in a state of igneous fusion, but have assumed a plastic condition by the intervention of water under great pressure and at a temperature far below that of fused lavas. They have hence been called by some geologists plutonic and by others hypogene rocks, the latter name signifying rocks generated beneath, in allusion to their obvious subterranean source. The distinctly stratified and sedimentary character of the great formations of crystalline rocks, and the obvious analogies which they present in this respect to the uncrystalline formations, early attracted the attention of geologists. In both occur intercalated layers of limestones, argillites, and conglomerates; and the question naturally arose as to the origin of the gneisses, mica schists, diorites, serpentines, chlorite schists, and talc schists, which are the characteristic rocks of these crystalline stratified formations. That the elements of these had in some way been deposited from water, like the beds of sand, mud, and carbonate of lime of uncrystalline strata, seemed obvious; and hence the conclusion that they were once, like the latter, uncrystalline strata, which had subsequently changed their form. In accordance with this notion, they were designated metamorphic strata, and this term is by many geologists used as synonymous with stratified crystalline rocks. It was noticed that in some instances uncrystalline sediments had assumed a crystalline character in the immediate vicinity of certain erupted rocks; the effect of heat, or more probably of the heated solutions impregnating the last, having generated in the midst of the contiguous sediments crystalline mineral species. It was then possible that a formation uncrystalline in one part of its distribution should elsewhere become crystalline, or in other words metamorphic; and it was conjectured that great areas of such rocks might be the stratigraphical equivalents of formations which are elsewhere uncrystalline sediments. In the Alps, for example, it was supposed that the gneisses and other crystalline schists were of mesozoic and even of cenozoic age, and similar rocks in other regions were declared to be palæozoic; till at length it seemed, such was the extension of the doctrine of rock metamorphism, that the sediments of any age might assume the characters of the primitive crystalline schists. In fact, the crystalline schists of the Alps, the British islands, and the Appalachians have all in turn been claimed as altered strata of palæozoic or more recent times. But these views have been controverted, and it has been shown that the crystalline strata which are now found in the Alps, superposed upon the uncrystalline fossiliferous sediments, are really ancient strata which were crystalline before the deposition of the latter, and in their normal position underlie them, but by great foldings and inversions have been brought to overlie them. In some instances in this region beds of apparently crystalline rocks are met with in which occur fossils like those of the uncrystalline sediments. These were regarded as further evidences of the metamorphic process which had proceeded so far as to develop a crystalline structure in the newer beds, without however obliterating their organic remains. But it has been shown that these pseudo-crystalline rocks are really sediments of the newer periods, made up of the ruins of the older and truly crystalline rocks. In many other cases, as in Wales and in eastern North America, it is found that the broken-up materials of the crystalline schists enter into the composition of the oldest palæozoic schists, which are themselves uncrystalline. While, therefore, it is clear that the crystalline schists were deposited from water, and, as will subsequently be seen, under conditions which, although chemically somewhat different from those of later times, did not prevent the development of organic life, it is now affirmed by one school of geologists that the great bodies of crystalline schists do not result from the alteration of any known series of uncrystalline strata; so that the division between the two established by Werner may still be retained as a fundamental one. This view is now sustained by Favre of Geneva, Sterry Hunt, Gümbel, Credner, and others; but the opposite view, which maintains a wide-spread metamorphism of palæozoic and more recent rocks, has been taught by very eminent names, and is still maintained in the principal geological text books and treatises. The partisans of the latter view, while asserting the comparatively recent origin of many crystalline schists, have always admitted the existence of an underlying or basal system of stratified crystalline rocks, which were supposed to be anterior in their formation to the appearance of life upon the earth, and from the apparent absence of fossils were called azoic rocks (signifying without life). In accordance with this nomenclature, the formations containing the fossil remains of plants and animals have been divided into palæozoic, mesozoic, and cenozoic rocks (signifying ancient, middle, and recent life); while subsequent discoveries, indicating that life had already made its appearance in the so-called azoic period, have led to the substitution of the name eozoic (signifying the dawn of life). These four great divisions are made the basis of the accompanying tabular view of geological formations. The subordinate divisions of Cambrian, Silurian, Devonian, &c., are of local origin, which, as will be seen, is also true of the names of most of the formations into which these in their turn are divided. In regard to the palæozoic rocks, which have been most minutely studied in Great Britain and America, the names of the subdivisions recognized in these countries are given side by side. For the details of the mesozoic and cenozoic rocks, which have been made the subject of not less careful analysis and subdivision in Europe, the reader is referred elsewhere. A complete table of them is given on page 109 of Lyell's “Student's Elements of Geology” (1871).


 Recent  Alluvial deposits, peat bogs, &c.
 Post-pliocene  Unstratified glacial drift, modified drift, &c.

Widely distributed along the eastern and southern coasts from Massachusetts to Texas, and from Nebraska across the continent to the Pacific.

 Upper cretaceous
 Lower cretaceous or Neocomian

Occurs in New Jersey, Georgia, Mississippi, Arkansas, &c., and from Texas and the upper Missouri in many localities westward to the Pacific.

 Upper, middle, and lower oölite

Widely developed in the western states in various localities from Dakota and Kansas to the Pacific.

 Triassic.  Upper, middle, and lower trias

Red sandstones of the Connecticut valley, New Jersey, Pennsylvania, the coal fields of Richmond, Va., and Chatham, N. C.

 Permian.  Magnesian limestone
 Permian Known in Illinois, Iowa, and Kansas.

 Carboniferous.   Coal measures
 Coal measures To this horizon belong the coal formations of New Brunswick, Rhode Island, Michigan, Illinois, and the great Appalachian coal field.
 Carboniferous limestone
 Millstone grit
 Lower carboniferous

 Upper, middle, and lower Devonian
 Portage and Chemung The Erie division of the New York series. Hence Dawson uses Erian as synonymous with Devonian.
 Upper Helderberg
 Schoharie and Cauda-galli.

 Upper and lower Ludlow

The upper Silurian of Murchison, the third fauna of Barrande. The stratigraphical and palæontological break at the top of the Water-lime makes two great divisions of the American Silurian.

 Lower Helderberg

 Onondaga or Salina
 Oneida and Medina
 Llandovery or May Hill  

 Caradoc or Bala  
 Hudson River

The lower Silurian of Murchison, or the second fauna of Barrande.


 Lingula flags

These include the primordial Silurian and the Cambrian of Murchison, the primal and auroral of Rogers, the Taconic of Emmons, and the Quebec group of Logan, and correspond to the first fauna or primordial zone of Barrande.

 Braintree and St. John's
 ——— ?
 ——— ?


 Primitive crystalline schists (Urschiefer
 Norian or Labrador

Above the Laurentian, and probably in the order here given.

 Montalban or White Mt
 Huronian or Green Mt
 Primitive gneiss (Urgneiss)
 Laurentian Dana uses the name Archæan as synonymous with Eozoic.

It should, however, be borne in mind that all such divisions of the rocks are arbitrary and artificial. From the mode in which sediments have been deposited, and from the alternations of sea and land, it follows that there are breaks in the succession of the rocks, which are often marked by a want of conformity in the arrangement of the successive formations. The sea retires from an uplifted continent, the strata become more or less disturbed, and perhaps in the course of ages partially broken down and swept away. When a new movement of the earth's crust brings this region once more beneath the sea, a new series of beds resting horizontally upon the older formation is deposited, and we have evidence, both from the relations of the strata and from the changes in the organic remains, of a break in the succession. Yet it is clear that elsewhere in the region occupied by the sea during this interval would be deposited sediments which fill up the interval. The process of deposition of sediments in the sea has never been interrupted, though the area of deposition has changed, and all breaks in the succession are local and accidental interruptions. Our divisions into systems and groups have been based in great part upon these interruptions, corresponding to omitted leaves in the succession, which the progress of investigation is now gradually supplying, so that the record when completed will show no breaks and no interruption either in the deposition of strata or in the succession of the forms of life. The disturbances or cataclysms which in the theories of the older school of geologists were looked upon as universal are really local, and are dependent upon the disturbances due to slow movements and the transfer of the process of sedimentation to other regions. But it is precisely where these breaks have been noticed that geologists have established horizons or lines of demarcation upon which the systems of classification have been built. From time to time we find out the formations which in other regions correspond to these interruptions, and serve to show the transition from one of the periods to another. These limits between hitherto separated formations are designated beds of passage. It is proposed to give a brief sketch of the successive geological groups enumerated in the preceding table, commencing with the lowest or eozoic period, and to notice the principal facts in their history, more especially as seen in North America.—The rocks which we have called eozoic include the crystalline strata, which are regarded in the present state of our knowledge as forming four great groups marked by lithological differences. At the base we have placed the Laurentian, which consists in great part of granitoid gneiss, in which, but for the interposed strata of quartzite, crystalline limestone, &c., there would in many parts be found small evidence of its stratified origin. This ancient group is what is called in Scandinavia the primitive gneiss, and corresponds to the fundamental granite which is often spoken of as underlying all other rocks. It is the oldest series of rocks known, and in North America forms a large part of the Laurentides, the Adirondacks, the Highlands of the Hudson, and their continuation southward. The thickness of this great series is unknown, but Sir William Logan has estimated that at least 20,000 ft. of strata belonging to it are exposed on the Ottawa river. It there includes three great limestone formations, which are associated with iron ore, plumbago, and phosphate of lime, and contain the remains of a foraminiferous organism to which Dawson has given the name of eozoon Canadense. To the Laurentian succeeds what has been named the Huronian, a group of crystalline rocks much more schistose than the Laurentian, and consisting of imperfect gneisses, with micaceous, chloritic, and talcose schists, and beds of hornblende and serpentine rocks, associated with argillites and magnesian limestones. This series is widely spread along both the N. and S. shores of Lake Superior, and the N. shore of Lake Huron, and constitutes the Green mountain range of eastern Canada and New England, stretching thence northeastward into Newfoundland and southwestward along the Appalachians. Rocks apparently belonging to this series fringe portions of the E. coast of New England, and are seen in a wider development in the coast range of southern New Brunswick. In some parts of the Lake Superior region the Huronian rocks are found to rest unconformably upon the Laurentian, and to be made up in part of its ruins, thus indicating a break between the two series. The third great group noticed in our table is that of the White mountains, or, as it may be called, the Montalban series. It consists in great part of gneisses, which, however, are lithologically dissimilar from those of the Laurentian, and are associated with large bodies of highly micaceous schists, abounding in kyanite, staurolite, andalusite, and garnet. This series of rocks is traced from the White mountains northeastward across the state of Maine and southwestward throughout the Appalachians. The facts, so far as known, seem to show that it is newer than the Huronian, resting unconformably upon it, and in some places probably upon the Laurentian in the absence of the former. The fourth group is what has been called the Norian or Labradorian, which consists in great part of granitoid or gneissoid varieties of the rock called norite, consisting chiefly of Labrador feldspar. With this are associated gneisses, quartzites, and crystalline limestones not unlike those of the Laurentian. This series in various parts of Canada and in northern New York appears to rest unconformably on the Laurentian, and was hence called by Sir William Logan the upper Laurentian; but according to recent observations by Hitchcock, it occurs in New Hampshire, apparently overlying the White mountain series. Dr. Sterry Hunt, who is the author of this attempt to group and classify the eozoic rocks, remarks: “The distribution of the crystalline rocks of the Norian, Huronian, and Montalban series suggests that they are remaining fragments of great formations once widely spread over an ancient floor of granitic (Laurentian) gneiss; but that these four series mentioned include the whole of the stratified crystalline rocks of North America is by no means certain. How many more formations may have been laid down over this region and subsequently swept away, leaving only isolated fragments, we may never know; but it is probable that a careful study may establish the existence of many besides the four series above enumerated.” Notwithstanding the distinction which has been drawn between crystalline and uncrystalline rocks, there is probably to be found somewhere a series of beds marking the passage from these crystalline schists to the uncrystalline sediments of the palæozoic, although, so far as yet studied, the oldest known strata hitherto referred to the latter are completely uncrystalline, and rest unconformably upon crystalline eozoic rocks. There appears to be a close similarity between the latter in widely separated countries, the great series already indicated being recognized with their typical characters in remote parts of the globe.—The palæozoic rocks have been divided into five great groups, sometimes called systems; but these divisions, as already remarked, are local, and the breaks in stratification and in the succession of organic remains are in some parts filled by beds of passage. As will be seen in the table, there is some difference in the nomenclature of the lower palæozoic rocks, a portion of the Cambrian of Sedgwick being included by Murchison in the Silurian. In the present account we shall use these terms in the sense in which they were applied by the former. The lower portions of the palæozoic show no evidence of terrestrial forms of life, their vegetable remains consisting of algae, and their animals of mollusks, corals, and crustaceans. At the summit of the Silurian, however, fishes and amphibians appear, while an abundant land vegetation of acrogens and gymnosperms begins to make its appearance. The palæozoic rocks are of especial interest to the student of American geology, as they form the surface of the greater portion of the United States east of the Rocky mountains. The succession of the members of the palæozoic series in this country was first clearly defined by the geological survey of New York, which in its reports in 1842 included under the name of the New York system the whole of the known palæozoic rocks to the base of the coal formation. The subdivisions then established have since been generally adopted in the United States, and their relations to those recognized in Great Britain will he seen in the table. The names Cambrian, Silurian, and Devonian found their way into American nomenclature some years later. For an account of the progress of discovery in these rocks, the reader is referred to the third part of a paper on “The History of Cambrian and Silurian,” by Dr. Hunt, in the “Canadian Naturalist” for July, 1872. The lower and middle Cambrian is represented in the New York series by the Potsdam sandstone, and the calciferous sand rock, having a combined thickness of less than 1,000 ft. To the eastward along the confines of New England, and thence northeastward along the base of the Green mountain range, however, a series of 10,000 ft. or more of sandstones, argillites, and limestones (including the Levis formation), is regarded as the representative of the lower and middle Cambrian, and has received the names of the Taconic system and the Quebec group. Still further east, along the E. coast, in Massachusetts, New Brunswick, and Newfoundland, are found strata of lower Cambrian age, referred to the Menevian of Great Britain. Between the middle and the upper Cambrian in New York is a break marked by a change in the fauna, and in some localities by a want of conformity between the strata. The Chazy limestone, which in some places is wanting, shows the passage between the two. The upper Cambrian is represented by the limestones of the Trenton group, followed by the Utica slates and the shales and sandstones of the Hudson river group; the last three divisions being known in Ohio as the Cincinnati group. Succeeding this occurs the Oneida conglomerate, followed by the Medina sandstone rocks, which are in part derived from the ruins of the underlying formations, and which mark a period of disturbance and a break in the succession. They are succeeded by the Clinton, Niagara, and Onondaga formations. The latter, sometimes known as the Salina formation, is characterized by beds of rock salt and of gypsum, and is succeeded by the water-lime beds, which, as well as the other strata of this division, from the Medina sandstone upward, consist chiefly of dolomite or magnesian limestone. This upper part of the American Silurian represents the deposits in a basin separated from the open ocean, and depositing by its gradual evaporation strata of salt and gypsum, the strata associated with which are almost destitute of organic remains. They attain a considerable thickness in Ontario and in central New York, but thin out to the eastward and disappear before reaching the Hudson river. To this division succeed the lower Helderberg limestones, characterized by an abundant fauna, and marking by their distribution a change in the geographical conditions of the region, by which a deposit of marine limestone was spread alike over all the preëxisting rocks, to the eastward, resting unconformably upon the Cambrian and the eozoic rocks, and attaining in eastern Canada a thickness of 2,000 ft. or more, where it is overlaid by a great series of sandstones, representing the Oriskany and the subsequent Devonian. This, in the New York series, is marked by but a small amount of sandstones, followed by the corniferous limestone and the Hamilton group, which together make up the upper Helderberg, and are succeeded by a series of sandstones, the whole constituting the Erie division of the New York series, the equivalent of the English Devonian or old red sandstone, and characterized by an abundant terrestrial fauna, the precursor of that of the carboniferous series, into which it passes by such transitions that it is a matter of discussion where to draw the line. The carboniferous series is so named because it is the earliest and most important coal-bearing series of strata, and includes great beds of fossil fuel, interstratified with sandstones and shales. At the base of the carboniferous in Michigan, Pennsylvania, and western Virginia, and also in Nova Scotia and New Brunswick, deposits of gypsum and salt are met with. In the western part of its distribution, toward the Mississippi, the carboniferous formation includes great thicknesses of marine limestone, which are wanting in the east. Overlying the carboniferous in Kansas and Iowa are beds which are the equivalent of the magnesian limestones of the north of England, and of the rocks called Permian in Russia. They are regarded as the summit of the palæozoic series.—The palæozoic rocks correspond to the transition rocks of Werner, to the lower part of which the name of the graywacke series was very generally given until the labors of Sedgwick and Murchison classified them and established the great divisions of Cambrian, Silurian, and Devonian. The thickness of these groups varies greatly in different parts of their distribution. Thus, while the entire palæozoic series in Pennsylvania is estimated at 40,000 ft., it is reduced to 4,000 in the valley of the Mississippi. This is due to the fact that the great sandstones, apparently derived from the erosion of rocks to the eastward, thin out in the opposite direction. In a similar manner the Cambrian and Silurian rocks, which attain in Great Britain a thickness of 30,000 ft., are represented by less than 2,000 ft. in Scandinavia.—Under the name of mesozoic or secondary rocks are included the triassic, Jurassic, and cretaceous series. The former has received its name from the threefold division of it in Europe into sandstones, overlaid by fossiliferous limestones, which are succeeded by sandstones and shales. At the base of the trias in the Tyrol, at St. Cassian and Haltstadt, occurs a series of fossiliferous beds in which the characteristic animal remains of the trias are found mingled with those of the palæozoic, thus showing a passage between the palæozoic and the mesozoic rocks. The trias, both in England and on the continent of Europe, is characterized by beds of rock salt and gypsum, like the Silurian and the lower carboniferous in North America. The sandstones of the trias in England are often red, and constitute what is there named the new red sandstone. The same name is applied to sandstones of similar age which are found in Prince Edward island and Nova Scotia, in the valley of the Connecticut, and in New Jersey, Pennsylvania, Virginia, and North Carolina. To this series belong the coal fields of Richmond, Va., and Chatham, N. C. It is not improbable that these beds may include strata belonging to the subsequent or Jurassic period, so named because it is greatly developed in the Jura mountains. This includes both the lias and the oölite of England, which two on the continent are connected by beds of passage known as the Koessen or Rhætic strata. The oölite of England consists of highly fossilfferous strata, chiefly marine, but in part fresh-water deposits, and through the Neocomian (Neufchâtel) beds passes into the cretaceous or chalk formation, the upper part of which is characterized in northern Europe by that pure uncrystalline limestone known as the chalk, a deep-sea deposit many hundred feet in thickness, made up almost entirely of the remains of minute animal organisms.—The rocks of the cenozoic or tertiary period are closely connected with the present time, and even in their lower portions contain some species of fossil shells identical with those now living. Lyell has conveniently divided the tertiary, in ascending order, into eocene, miocene, and pliocene; to these are added a postpliocene division which includes the period of glacial drift. (See Diluvium.) The tertiary rocks attained a great thickness in some parts of their distribution. Thus in the Alps the miocene sandstones and conglomerates, known as the molasse, have in parts a thickness of more than 6,000 ft., while the nummulitic limestone, a subdivision belonging to the base of the tertiary, attains in the Mediterranean basin a thickness of more than 2,000 ft.—We have already spoken of the trias of the eastern part of North America. The cretaceous is also represented in New Jersey and along the southern border of the palæozoic from Georgia to Tennessee. Triassic, Jurassic, and cretaceous rocks are also widely spread between the Mississippi and the Rocky mountains, from Texas to Dakota, and westward over large areas to the Pacific coast. Deposits like the English chalk are unknown in this formation in North America. Tertiary rocks of various ages skirt the Atlantic coast from the Rio Grande to New Jersey, and are even met with off the coast of Massachusetts. They stretch from the gulf of Mexico to Kentucky, and like the mesozoic rocks occupy large areas to the westward, where on the Pacific coast they attain great thickness.—The succession of organic life in these various groups constitutes a study by itself, which will be considered under the head of Palæontology. The palæozoic age is preeminently the period of mollusks, corals, and crustaceans, the most important class of which last in the early times were the trilobites, which appear in their greatest development in the Cambrian and Silurian, and die out in the carboniferous. Fishes, the earliest representatives of vertebrate life, make their appearance near the summit of the Silurian, and abound in the upper palæozoic; reptiles first appear in the carboniferous, and reach their greatest development in the mesozoic, in which reptilian forms of immense dimensions, and having curious resemblances to birds, are met with; while the birds themselves, which then first appeared, had remarkable reptilian affinities. The earliest evidences of mammals appear in the trias; throughout the mesozoic they were insignificant in size, and chiefly marsupial. In the eocene and miocene divisions of the tertiary we find the greatest development of mammalian forms. The deposits of these strata to the west of the Mississippi have within the last few years afforded a great number of remarkable species of mammals, which have been described by Leidy, Marsh, and Cope. The flora of the tertiary period is not less remarkable than its fauna. The geographical and climatic conditions of the northern hemisphere were then widely different from those of the present day. Not only over Europe, but in North America, and northward as far as Greenland and Spitzbergen, a mild and equable climate prevailed, and the abundant plant remains preserved in the tertiary beds of those arctic regions show a luxuriant vegetation like that of the warmer parts of the temperate zone of to-day. This condition of things had been of long continuance; for in western America great beds of coal or lignite are found both in the cretaceous and the eocene strata. It was continued far into the pliocene; but as this went on, a cold climate like that which now characterizes the northern hemisphere prevailed, and gave rise to the glacial phenomena which have been described under the head of Diluvium. This change of climate is one of the most perplexing problems of geology. That a different distribution of land and water and of the oceanic currents may have contributed in some degree to this former climatic condition of the arctic regions is probable. Astronomical conditions connected with changes in the eccentricity of the earth's orbit have also been suggested as a cause; and finally it has been supposed that a somewhat different chemical composition of the earth's atmosphere prevailing up to that time may have coöperated with geographical conditions to maintain the peculiarly mild climate which, so far as we can judge, prevailed throughout the arctic regions in palæozoic times, and perhaps without interruption nearly to the close of the tertiary.—The distribution of metallic ores and other economic materials in the various geological series is a point of much interest, and demands a brief notice in this place, although the subject is discussed more in detail under Mineral Veins, and in the articles on the different metals. Metallic ores are met with both in beds interstratified with the rocky layers and in veins cutting these. The eozoic rocks are remarkable for their great deposits of crystalline iron ores, of which those of the Laurentian on Lake Champlain and those of the Huronian on Lake Superior are remarkable examples, as are also those of Missouri. Similar deposits occur in the eozoic rocks of Scandinavia and Russia. It is in these rocks also that titanic and chromic iron and emery occur; and to them belong graphite and beds of iron pyrites and copper pyrites, often associated with gold and with silver. Oxide of tin also appears to be characteristic of these crystalline rocks. These various ores are found not only in contemporaneous layers, but also in veins and beds cutting the crystalline strata. But the metallic ores are not confined to these more ancient rocks, for beds of oxide and carbonate of iron are met with at various horizons from the Cambrian up to recent times, while under the heads of Copper and Gold the distribution of those metals and their ores is described. Besides these contemporaneous deposits, veins or lodes carrying the ores of various metals are found cutting rocks of all ages, and are probably even now in process of formation.—The question of eruptive or exotic rocks has already been briefly alluded to, but from its intimate connection with volcanic phenomena, from which it cannot well be separated, it is proposed to consider the whole subject in the article Volcano, in which connection the various theories with regard to the nature of the earth's interior, the sources of subterranean heat and of ancient and modern eruptive rocks, as well as of the gaseous products of volcanic eruptions, will be discussed. (See also Granite.) Under the head of Mountain will be considered some of the most important questions of geological dynamics, namely, those relating to the elevation of continents, the phenomena of denudation, and the origin of mountains. The chemical history of the globe, or what may be called chemical geology, will be discussed under the titles Rocks and Water.