Popular Science Monthly/Volume 68/April 1906/The Development of the Glacial Hypothesis in America
By Dr. GEORGE P. MERRILL,
U. S. NATIONAL MUSEUM
GEOLOGY is preeminently a science of observation and deduction. Certain phases of it are, nevertheless, dependent upon advances in other branches. As the science grows this mutual interdependence becomes more and more apparent, and it is perhaps now questionable if further advance, aside from a purely geographic extension of knowledge relative to the distribution of geologic groups, is possible without calling in the aid of physics and chemistry.
Among those phases of geology which have been most independent of the allied sciences, and in which the gradual development of the power to observe correctly and deduce accordingly can incidentally be readily studied, the phenomena of the drift stand prominent. They furnish, moreover, an excellent example of the growth of knowledge through cumulative evidence, since of all phenomena, those relating to the drift have, in America, been perhaps longest under observation. In that which follows, however, it has been my principal aim to trace the gradual progress of the glacial hypothesis in America. Beyond this my references are purely incidental.
Out of numerous observations relating to the phenomena of the drift and, incidentally, to glaciation, those which seem most worthy of consideration at the present time were by Benjamin De Witt and are to be found recorded in the second volume of the 'Transactions' of the Philadelphia Academy for 1793.
De Witt noted the occurrence, along the shores of Lake Superior, of boulders representing a large variety of rocks, sixty-four different types being collected. In the discussion of this occurrence, he remarks:
Now, it is almost impossible to believe that so great a variety of stones should be naturally formed in one place. . . . They must, therefore, have been conveyed there by some extraordinary means. I am inclined to believe that this may have been effected by some mighty convulsion of nature, such as an earthquake or eruption, and perhaps this vast lake may be considered as one of those great 'fountains of the deep' which were broken up when the earth was deluged with water, thereby producing that confusion and disorder in the composition of its surface which evidently seems to exist.
This, so far as I am aware, is the first attempt to account for the wide dispersion of drift boulders over the northern part of the country, although the glacial character of the same was not dreamed of.
The paper, so far as existing literature shows, caused little discussion, and we have next to refer to observations by Dr. Samuel Akerly, who, in 1810, published in Bruce's American Mineralogical Journal a geological account of Dutchess County, New York. After referring to the Highlands and the country to the northward, he described the southern part—that upon which the then existing city was built—as composed of an alluvion of sand, stone and rocks. This he looked upon as a recent deposit, 'subsequent to the creation and even the deluge.' The manner in which this material was deposited he described in the following language:
After the waters of the deluge had retired from this continent, they left a vast chain of lakes, some of which are still confined within their rocky barriers. Others have since broken their bounds and united with the ocean. The Highlands of New York was the southern boundary of a huge collection of water, which was confined on the west by the Shawangunk and Catskill mountains. The hills on the east of the Hudson confined it there. When the hills were cleft and the mountains torn asunder, the water found vent and overflowed the country to the south. The earth, sand, stones and rocks brought down by this torrent were deposited in various places, as on this island, Long Island, Staten Island and the Jerseys. This opinion, he added, is mostly hypothetical, because unsupported by a sufficient number of facts.
A candid acknowledgment upon which the author is to be congratulated!
"Whose was the master mind that first conceived of this great barrier which held back the flood of waters, so long made responsible for the drift, I have not been able to ascertain. The theory is, however, given in greatest detail by Dr. Samuel L. Mitchill in his 'Observations on the Geology of North America,' published in 1818.
It was Mitchill's idea that the Great Lakes are the shrunken representatives of great internal seas of salt water, which ultimately broke through their barriers, the remains of which he thought to be still evident. One of them he wrote, seemed to have circumscribed the waters of the original Lake Ontario and to be still traceable as a mountainous ridge beyond the St. Lawrence in upper Canada, passing thence into New York, where it formed the divide between the present lake and the St. Lawrence and continued to the north end of Lake George, apparently crossing the Hudson above Hadley Falls. Thence, he believed it to run toward the eastern sources of the Susquehanna, which it crossed to the north of Harrisburg, and continued in a southeasterly direction until it entered Maryland, passing the Potomac at Harpers Ferry into Virginia, where it became confounded with the Allegheny Mountains.
To appreciate Mitchill's views, then, we have to imagine this now broken and gapped ridge as continuous. A time came, however, when at various points it gave way, the pent-up waters rushing through and carrying devastation before them like the waters from cloud-burst or bursting reservoirs of to-day, but on a thousandfold larger scale. By this bursting all the country on both the Canadian and Fredonian sides must have been drained and left bare, exposing to view the water-worn pebbles, and the whole exhibition of organic remains there formed. Great masses of primitive rocks from the demolished dam, and vast quantities of sand, mud and gravel were carried down the stream to form the curious admixture of primitive with alluvial materials in the regions below.
A fresh contribution to the subject was rendered this same year, in the publication of Amos Eaton's 'Index to the Geology of the Northern States.' Eaton's views were in part, at least, a reflection of those of Werner. We have to do here, however, only with that portion of the work relating to the so-called alluvial class of rocks. In discussing this and attempting to account for the great masses of granite and syenite which he found scattered throughout the Connecticut River region, he wrote:
What force can have brought these masses from the western hills, across a deep valley seven hundred feet lower than their present situation? Are we not compelled to say that this valley was once filled up so as to make a gradual descent from the Chesterfield range of granite, syenite, etc., to the top of Mount Tom? Then it would be easy to conceive of their being rolled down to the top of the greenstone where we now find them.
It was not easy in all cases for the geologists of these early days to distinguish between the younger and earlier drift, or between the material which we now consider as glacial drift and the loosely consolidated alluvial deposits of the Tertiary period. This seems particularly true of Dr. H. H. Hayden, a Baltimore dentist and one-time architect, who in 1820 published a volume of geological essays in which he dwelt very fully upon the lowlands, or the area at present comprised within the so-called Coastal Plain. After referring to the geographical limits of this plain and combating the opinions of previous observers, he elaborated his own theories somewhat as follows:
Viewing the subject in all its bearings, there is no circumstance that affords so strong evidence of the cause of the formation of this plane as that of its having been deposited by a general current which, at some unknown period, flowed impetuously across the whole continent of North America in a northeast and southwest direction, its course being dependent upon that of the general current of the Atlantic Ocean, the waters of which were assumed to have risen to such a height that it overran its limits and spread desolation on its ancient shores.
In seeking the cause of this general current Hayden referred first
to the seventh chapter of Genesis:
For yet seven days and I will cause it to rain upon the earth forty days and forty nights, and every living substance that I have made will I destroy from off the face of the earth.
He then proceeded to show the inadequacy of rainfall alone, since the water being thus equally distributed over the ocean and the land, there could be no tendency toward a current. Some other cause must be sought, and, fortunately, his imagination proved equal to the task.
Accepting as probable the suggestion of one whom he designates as ' a writer of no common celebrity,' to the effect that the cause of the general deluge was a melting of the ice at the two poles of the earth, he proceeded to explain in his own way the details of the catastrophe, though acknowledging that no positive testimony could possibly be adduced to substantiate the fact.
Having admitted the possibility of the earth's changing its position, so that the sun would pass immediately over the two poles on an unknown meridian, he showed that there would then result a rapid dissolution of the existing ice-caps such as would yield an ample supply of material, it being only necessary to give it direction. Considering as essential to the problem only the northern hemisphere, he remarks that, from this polar cap there were but two outlets, the one into the Pacific through the narrow Bering Straits, and the other through the wider channel between Greenland and Lapland into the Atlantic. Hence, when the melting ensued, by far the larger volume of water passed into the latter ocean. No sooner was this operation established and this accession of strength and power thrown into the Atlantic Ocean in particular, than its tide began to rise above its common limits, accompanied by a consequent current, both constantly increasing, the one in height and the other in rapidity.
At the commencement of this frightful drama the current, it is highly probable, was divided by the craggy heights of Spitzbergen and a part thrown into the White Sea, while the other was thrown back upon the eastern and southern coast of Greenland and thence in a southwestern direction until it struck the southern coast of Labrador, along which it swept, through the straits of Belle Isle, across Newfoundland, Nova Scotia, and along the Atlantic coast into the Gulf of Mexico. In a short space of time the southern and eastern coast of Labrador was desolated. The soil was hurled adrift and carried across the country into the Gulf of St. Lawrence and across a part of New England into the sea or general current of the ocean.
Continuing to rise, the waters spread across Davis Straits and rolled their tumultuous surges into Hudsons Bay, embracing the whole coast of Labrador, while the current of the St. Lawrence was forced back and upward to its parent source.
At length the floods of the pole, forming a junction with Baffins Bay and the Arctic Sea, defying all bounds, overran their ancient limits and hurled their united forces in dread confusion across the bleak regions of the north to consummate the awful scene. Thus, lakes and seas uniting, formed one common ocean which was propelled with inconceivable rapidity across the continent between the chains of mountains, into the Gulf of Mexico, and probably over the unpeopled wilds of South America into the southern ocean. Fulfilled in this way were the awful denunciations of an offended God, by the sure extermination of every beast of the field and every creeping thing that crawled upon the face of the earth.
To these causes he believed to be due, not merely the deposits of the coastal plain, but, as well, the barrenness of Labrador and the northeastern portion of the continent, and the general phenomena of the glacial drift, the boulders of the latter being conceived as transported by floating ice.
These essays, it may be noted, were favorably reviewed by Silliman in the third volume of his Journal, even the idea of the fusion of the polar ice-cap being allowed to pass with no more serious criticism than that the flood of waters might have been produced through the expulsion of the same from cavities in the earth. J. E. DeKay, however, writing some years later, ventured to take exception to the views regarding drift boulders, wisely suggesting that, since the speculative part of geology is but a series of hypotheses, we should in every case admit that which explains the phenomena in the simplest possible manner. To his mind the simplest manner of accounting for these boulders was to suppose that such had been, as igneous material, extruded through the superincumbent strata, forming peaks which have since been destroyed through some convulsion of nature or through the resistless tooth of time, the boulders thus being fragments which had escaped destruction, though their place of extrusion had become completely obscured.
The observations thus far recorded display a lack of close attention to details and, in some cases, indicate a decided leaning towards cosmogony. Those to which I now call attention were of quite different type and show their author to have' been a man of more than ordinary discernment.
While superintending excavations preliminary to the erection of a cotton mill at Vernon, Conn., Mr. Peter Dobson observed the boulders in what we now call the ground moraine or till, and in a letter dated November 21, 1825, described them as worn smooth on their under side as if done by having been dragged over rocks and gravelly earth in one steady position. They also showed scratches and furrows on the abraded parts. These appearances he could account for only by assuming that the blocks had been worn by being carried in ice over rocks and earth under water.
These observations seem to have attracted no attention at the time, and even Dr. Edward Hitchcock thirteen years later attached no serious importance to them, although his attention was called to the matter by another letter from Dobson, this time addressed to himself. In this last letter, written in 1838, Dobson described the boulders as having first been rounded by attrition and then worn flat on one side by a motion that kept them in one relative position, as a plane slides over a board in the act of planing. Some of them were described as worn and scratched so plainly that there was no difficulty in pointing out which side was foremost in the act of wearing, a projecting bit of quartz or feldspar protecting the softer material behind it. In this letter he again announced his inability to account for the appearances except on the supposition that they had been enveloped in ice and moved forward over the sea bottom by currents of water. The drifting icebergs of the Labrador coast he thought might well illustrate the conditions of their production.
Perhaps it may have been because Dobson was a cotton manufacturer and not a member of one of the learned professions, or there may have been other reasons, but Hitchcock allowed the observations to pass unnoticed until 1842, when the subject was brought up by Sir Eoderick Murchison in his anniversary address before the Geological Society of London.
I take leave of the glacial theory in congratulating American science in having possessed the original author of the best glacial theory, though his name has escaped notice, and in recommending to you the terse argument of Peter Dobson, a previous acquaintance with which might have saved volumes of disputation on both sides of the Atlantic.
Supported by this somewhat enthusisastic endorsement, Hitchcock then gave the letter to the public through the American Journal of Science, at the same time remarking that he had himself derived his ideas concerning the joint action of ice and water from the writings of Sir James Hall.
With this much in the way of anticipation, I will turn back to 1825 once more and refer to the writings of William Keating, mineralogist, who accompanied Major Long's expedition to the sources of the St. Peters River. This observer noted that the entire region of the present headwaters of the Winnipeek River had been at a comparatively recent period an immense lake, interspersed with innumerable barren rock islands, which had been drained by the bursting of the barriers which tided back the waters. This was plainly a recognition of the now extinct glacial Lake Agassiz.
Although the cosmogonist was fast drifting into the obscurity of the past, there were, nevertheless, occasional writers who preferred to ignore facts of observation or the efficiency of simple causes, and to seek for more difficult or more mystical methods of accounting for phenomena than those afforded by the observation of processes now in action. Thus, Benjamin Tappan, in discussing in 1828 the boulders of primitive and transition rocks found in Ohio, objected to the commonly accepted idea that such were necessarily foreign to the locality and brought by currents of water or floating ice. He frankly acknowledged, however, his own inability to account for their presence, but ingenuously claimed that
Ignorance is preferable to error. It may, therefore, be asked why may not these rocks have been created where they are now found, or, again, why may they not have been thrown up by earthquakes or volcanoes?
Groping though this writer may have been, it is questionable if his ignorance was not preferable to the kind of knowledge manifested by a writer in the American Journal of Science two years previously, who had accounted for the drift on the supposition that the earth's revolution, amounting to 1,500 feet a second, was suddenly checked. This, he thought, would result in the whole mass of the surface water rushing forward with inconceivable velocity until overcome by opposing obstacles or exhausted by continual friction and the counterbalancing power of gravitation. The Pacific Ocean would thus rush over the Andes and the Alleghenies into the Atlantic, which would, in the meantime, be sweeping over Europe, Asia and Africa.
A few hours would cover the entire surface of the earth, excepting, perhaps, the vicinity of the poles, with one rushing torrent in which the fragments of disintegrated rocks, earth, and sand would be carried along with the wreck of animal and vegetable life in one all but liquid mass.
The first geological survey of an entire state carried through at public expense was that of Massachusetts, authorized by legislative enactment in 1830. Dr. Edward Hitchcock, then professor of chemistry and natural history in Amherst College, was selected to carry out the work. The report presented early in 1832 was, therefore, a document of unusual importance and, to a certain extent, epoch-making. Much that is of interest is to be found within its pages, but we must limit ourselves to that relating purely to the distribution of the numerous erratics for which the state is noted.
It is but natural that this drift should have been attributed to the Noachian deluge, when one considers that Hitchcock's training was that of a clergyman. Speaking of that about Cape Ann, he wrote:
It can not fail to impress every reasoning mind with the conviction that a deluge of tremendous power must have swept over this cape. Nothing but a substratum of syenite could have stood before its devastating energy.
This observation is of importance, since here, for the first time, Hitchcock put himself on record in a line of investigation in which he became more widely known than in any other, with the possible exception of that relating to the fossil footprints of the Connecticut Valley.
In 1836 there was established a state geological survey of Maine, with C. T. Jackson, of Boston, at its head. Jackson's views on the glacial deposits, as expressed in the annual reports, were perhaps not more crude than those of the average geologist of his day. The 'horsebacks' (ridges of glacial gravel) were regarded by him as of diluvial material, transported by a mighty current of water which, he supposed, rushed over the land during the last grand deluge, accounts of which had been handed down by tradition and preserved in the archives of all people. "Although," he says, "it is commonly supposed that the deluge was intended solely for the punishment of the corrupt antediluvians, it is not improbable that the descendants of Noah reap many advantages from its influence, since the various soils underwent modifications and admixtures which render them better adapted for the wants of man. May not the hand of benevolence be seen working even amid the waters of the deluge?" he asks. It is, perhaps, doubtful if the hard-fisted occupants of many of Maine's rocky farms would be disposed to take so cheerful a view of the matter.
Substantially the same views were advanced by Jackson in his report on the geology of Khode Island, which appeared in 1839.
There can not remain a doubt but that a violent current of water has rushed over the surface of the state since the elevation and consolidation of all the rocks and subsequent to the deposition of the Tertiary clay, and that this current came from the north. . . . Upon the surface of solid ledges, wherever they have been recently uncovered of their soil, scratches are seen running north and south, and the hard rocks are more or less polished by the currents of water which at the diluvial epoch coursed over their surfaces, carrying along the pebbles and sand which effected this abrasion, leaving stria, all of which run north and south, deviating a few degrees occasionally with the changes of direction given to the current by the obstacles in its way.
He did not accept the theory of drifting icebergs; 'nor,' he wrote, ' can we allow that any glaciers could have produced them by their loads of sliding rocks, for in that case they should have radiated from the mountains instead of following a uniform course along hillsides and through valleys.'
Although primarily a paleontologist, Timothy Conrad was sometimes drawn out of his chosen field by phenomena too obvious to be overlooked and concerning the nature of which little was actually known by the best authorities. The occurrence of enormous boulders in the drift, resting often upon unconsolidated sand and gravel, fell within this category. That such could not have been brought into their present position through floods was to him obvious, neither could they have been floated by ice floes from the north during a period of terrestrial depression. He assumed, rather, that the country previous to what is now known as the glacial epoch was covered with enormous lakes, and that a change in climate ensued, causing them to become frozen and converted into immense glaciers. At the same time elevations and depressions of the earth's surface were in progress, giving various degrees of inclination to the frozen surfaces of the lakes, down which boulders, sand and gravel would be impelled to great distances from the points of their origin. The impelling force, he thought, in some cases might be gravity alone, but during the close of the epoch, when the temperature had risen, vast landslides—avalanches of mud filled with detritus—would be propelled for many miles over these frozen lakes, and when the ice disappeared, the same would be deposited in the form of a promiscuous aggregate of sand, gravels, pebbles and boulders.
In 1840 an immense stride in the study of drift deposits was made through the publication of Louis Agassiz's 'Etude sur les Glaciers,' a work comprising the results of his own study and observation combined with those of Jan de Charpentier, E. T. Venetz and F. G. Hugi. The work was published in both French and German, and brought to a focus, as it were, the scattered rays by which the obscure path of the glacial geologist had been heretofore illuminated. It was Agassiz's idea, as is well known, that at a period geologically very recent, the entire hemisphere north of the thirty-fifth and thirty-sixth parallels had been covered by a sheet of ice possessing all the characteristics of existing glaciers in the Swiss Alps. Through this agency he would account for the loose beds of sand and gravel, the boulder clays, erratics, and all the numerous phenomena within the region described, which had been heretofore variously ascribed to the Noachian deluge, the bursting of dams, the sudden melting of a polar ice-cap, or even to cometary collisions with the earth. These ideas were favorably received by the majority of workers, though there was, naturally, a highly commendable feeling of caution against their too hasty acceptation. As a reviewer in the American Journal of Science expressed it:
These very original and ingenious speculations of Professor Agassiz must he held for the present to be under trial. They have been deduced from the limited number of facts observed by himself and others and skillfully generalized, but they can not be considered as fully established until they have been brought to the test of observation in different parts of the world and under a great variety of circumstances.
The effect of this publication, however, soon made itself apparent in the current literature. Thus, in 1843 Professor Charles Dewey, writing on the striae and furrows on the polished rocks of western New York, argued that, while the boulders of the drift indicated that a mighty current had swept from north to south, the polishing and grooving might be due to glaciers. 'Glaciers or icebergs and the strong currents of water—a union of two powerful causes—probably offer the least objectionable solution of those wonderful changes,' he wrote. Though thus disposed to accept in part Agassiz's conclusions, Dewey yet failed to realize their full possibilities.
He could not conceive how it was possible for a glacier to transport sandstone boulders from the shore of Lake Ontario to the higher level of the hills to the southward. Boulders of graywacke removed from the hills in the adjoining part of the state of New York and scattered throughout the Housatonic Valley furnished a like difficulty, since between the place of origin and that of deposit lay the Taconic range of mountains. "If," he wrote, "the boulders were once lodged on the glacier, the ice and boulders must have been transported by a flood of waters over the Taconic Mountains."
In 1842 Dr. Edward Hitchcock, already referred to in connection with Dobson's work, read an important paper before the Boston meeting of the American Association of Geologists on the phenomena we are now discussing, which is particularly interesting as showing the gradual evolution of the present theory from that of the Noachian deluge idea, advocated by the earlier workers. His views had evidently been modified by the publication of Agassiz's work and by the writings of Buckland, Murchison, Dobson and others, and he expressed at the outset the conviction that nearly all geologists would agree in the principle that the phenomena of drift were the result of joint and alternate action of ice and water. To express this joint and alternate action he made use of the term aqueo-glacial.
In this paper Hitchcock devoted some fifty pages to a description and discussion of observed phenomena, after which he proceeded to discuss the theories of the various European authorities and state his agreement or objections to the same. He objected to the theory of Lyell, to the effect that the results observed by him in North America were produced by floating icebergs derived from glaciers formed on mountains as the land gradually emerged from the ocean, because, first, it failed to account for the lower temperature which was necessary; second, because there was no evidence that the glaciers descended from the mountains; and third, because the deposits of vegetable matter derived from land plants showed that the continent must have been above sea-level long before the drift period.
The theory of De la Beche, which supposed the contents of the northern ocean to have been precipitated over the countries further south by the elevation of the polar regions, Hitchcock regarded as possibly applicable to the low countries of Europe, but not to New England, since it would require a rise of the ocean amounting to some 6,000 feet, and he could find no facts to justify such an assumption, although recognizing the fact that the aqueo-glacial agency had operated well over the summits of the White Mountains.
To Agassiz's theory, which supposed an immense accumulation of ice and snow around the poles during the glacial period and a consequent sending out of enormous glaciers in a southern direction, followed by enormous floods of water and transportation of icebergs on return of a warmer period, he likewise took exception, since he was unable to conceive how such effects could be brought about. Nor, indeed, could he understand how such causes could operate when the land was rising from the ocean and the water consequently retreating, as it must have been to account for the various observed phenomena—such phenomena as would necessitate the occurrence of water loaded with ice and detritus floating for centuries at least over a large part of the earth's surface.
His paper showed a very clear insight into what had taken place, but an inability, with the information at that time available, to account for it in a satisfactory manner. Thus, in describing the striae found by himself on the top of hills and mountains like Monadnoc, he wrote:
Could immense icebergs have been stranded on the northern slope of the hills and afterwards, by the force of currents, have been driven over the summits; or would it be necessary to suppose that, after the stranding, the water must have, risen so as to lift the iceberg; or would a vast sheet of ice lying upon the earth's surface, by mere expansion, without the presence of water, have been able to produce the smoothing and furrowing in question?
After considering the phenomena and weighing all the theories advanced from time to time by the authorities quoted, he summed up the matter in the following words:
Is it not possible that the phenomena of the drift may have resulted from all the causes advanced in the theories under consideration? I feel . . . that the proximate cause of the phenomena of drift has at last been determined, namely, the joint action of water and ice.
In 1836 there was organized a state geological survey of New York, which was placed in charge of W. W. Mather, Ebenezer Emmons, Lardner Vanuxem and James Hall—men whose names have since become too thoroughly identified with American geology to ever become eradicated. Naturally drift phenomena attracted the attention of these workers, and each expressed opinions, some of which may be referred to in detail.
Seventy-five pages of Mather's report, as published in 1843, were given up to descriptions and discussions of drift phenomena. He concluded that the transport of the material and the production of scratched surfaces were contemporaneous, the drift itself being transported in part by currents and in part by ice itself drifted by the currents. The period of the drift and that of the quaternary deposits were separated by a partial submergence of the land, and, further, the periods of the drift were periods when the currents were stronger than at the present time. He conceived this to be due to a collapse of the crust of the globe upon its nucleus, causing an acceleration of the velocity of rotation, and this causing, in its turn, a disturbance of the form of equilibrium of the spheroid of rotation which had been compensated by the flow from the polar regions and an accelerated flow to the equatorial regions. This sudden acceleration of the ocean currents he felt would be sufficient to cause the transportation of vast quantities of detritus-laden ice from the polar regions southward. The large amount of drift scattered over the central and northern Mississippi region he ascribed incidentally to ice-laden currents from Hudsons Bay and the polar seas, which, floating over the northern part of the United States, would be met by the warm waters of the Gulf Stream, causing them to deposit their loads. The warm current flowing northward would be superimposed on the cold current, the latter continuing southward beneath it, transporting the finer materials, such as now occupy the lower Mississippi Valley.
Emmons likewise believed the agent of drift transportation to be water and ice. The boulders he thought to be the work of icebergs, but the striations and polishing he felt could not be due to this agency, since the bottom of the ocean is not bare rock, but covered by debris, and, moreover, icebergs would not move in straight lines, a point which some more recent writers have quite overlooked. The bergs might act as agents of transportation, he argued, but not of erosion. According to his ideas the drift-covered region was, during the drift period, depressed, the country low and connected at the north with an extensive region giving rise to large rivers which flowed in succession over different parts of the region lying between Champlain and the St. Lawrence. These rivers united with the Atlantic on the south through the Champlain, Hudson and Mohawk valleys. They bore along ice loaded with sand, pebbles, etc., which scratched and grooved the surface of rocks over which they flowed, and were the agents also of perforating the rocks in the form of pot holes.
Hall's ideas were somewhat hazy. That he did not accept Agassiz's doctrine of a vast ice sheet is very evident. Thus, he wrote that
Blocks of granite, either enclosed in ice or moved by other means, have been the principal agents effecting the diluvial phenomena; that they have scored and grooved the rocks in their passage and, breaking up the strata and mingling themselves with the mass, have been drifted onward carrying everything before them in one general melee. That such may have been the case in some instances or in limited localities, can not be denied; but that it ever has been over any great extent of country will scarcely admit of proof.
Hall was at this time evidently a catastrophist and regarded the drift soils, terraces, and the deep valleys and water courses as due to the violent action of water which may have been caused in part by a sudden submergence and the rapid passage of a wave over its surface. His views, indeed, were in many respects little, if any, in advance of those held by Mitchill twenty-five years earlier. Like Mitchill, he conceived of an inland sea bounded and held back by the Canadian highlands on the north, the New England range on the east, and highlands of New York and the Alleghenies on the south, and the Rocky Mountains on the west. These presented barriers of from one thousand to twelve hundred feet above the level of the ocean until broken through by the St. Lawrence, the Susquehanna, the Hudson, partially by the Mohawk at Little Falls, and perhaps also by the Connecticut.
But to whatever cause we attribute the phenomena of the superficial detritus of the fourth district, the whole surface has been permanently covered by water, for it seems impossible that partial inundations could have produced the uniform character and disposition of the materials which we find spread over the surface.
Under the caption 'Description of a Singular Case of Dispersion of Blocks of Stone Connected with the Drift, in Berkshire County, Mass.,' Dr. Edward Hitchcock came forward in the American Journal of Science for 1845 with a description of that remarkable train of boulders extending from Fry's Hill in the Canaan Mountains of New York, southeasterly into Massachusetts for a distance of some fifteen or twenty miles, which have since become more generally known as the Richmond Boulder Train. The lithological nature of the boulders was such that they could be traced to a common source and were described as forming three somewhat meandering trains, extending from Fry's Hill, through the adjoining valley, and upwards over an elevation of eight hundred feet at the state line, across the Richmond valley, over Lenox Mountain, six hundred feet in height, to and over Beartown Mountain, one thousand feet in height. Naturally, so striking a phenomenon excited investigation, and, naturally, too, Dr. Hitchcock, in the then existing condition of knowledge, found difficulty in accounting for the same. He recognized the similarity of the trains to the lateral moraines described by Agassiz, but he could not conceive of a glacier traveling directly across the intervening ridges, even were there mountains in the vicinity of sufficient altitude to give rise to the same. Neither did the consideration of river drift or floating ice afford him a satisfactory conclusion:
In short, I find so many difficulties on any supposition which I may make that I prefer to leave the case unexplained until more analogous facts have been observed.
Unsatisfactory and apparently unimportant as this paper may at first thought seem, it is questionable if the contribution were not worthy of greater commendation than the one put forward three years later by the Rogers brothers, to which I now refer.
According to the descriptions given, the trains start, each from its particular depression in the summit of a high ridge in Canaan, N. Y. Taking a direction south 35° east, they cross the higher ridges and their intervening valleys, the longer for a distance of twenty miles and the shorter for ten miles. The individual trains are none of them more than three hundred or four hundred feet in breadth and not over half a mile asunder. The transported blocks of all sizes up to twenty feet in diameter, sharply angular, free from scratches, and all of the same lithological nature, identical with that of the ridge whence they start. That such a dispersion of boulders from a single point should have taken place regardless of contours is certainly enough to excite the interest of any one. It is the means invoked by the two workers which have excited our wonder, however.
After exhibiting to their own satisfaction the inadequacy of either the iceberg or the glacial hypothesis to account for their production, the authors attempted to show how all the phenomena might be explained by the theory of a sudden discharge of a portion of the Arctic Ocean southward across the land. They discussed the important functions of the 'wave of translation,' showing its surpassing velocity and great propulsive power, and traced the influence of vehement earthquakes near the pole in dislodging the northern waters and ice and maintaining in the rushing flood these vast and potent waves. They then suggested that, at a certain stage of the inundation, the ice, previously floating free, might impinge with irresistible violence against the tops of submerged hills, and that the Canaan Mountain stood precisely in the position to take the brunt of the ice-driving flood as it swept down the long, high slope of the distant Adirondacks and across the low, broad valley of the Hudson.
They then proceeded to show that, at the instant when some enormous ice island struck the crest of the mountain and scooped the trench which we there behold, a great vortex was produced by the obstruction thus suddenly thrown in the path of the current, which, endowed with an excessive gyratory or spiral velocity, was capable of sustaining and carrying forward the greater part of the fragments. As in the instance of the waterspout and the whirlwind, the whirlpool would gather into the rotating column the projected blocks and strew them in a narrow path in the line along which its pendent apex would drag the ground.
Truly there were catastrophists in those days!
Agassiz, it will be remembered, came to America in 1846, and in 1847 was appointed to the professorship of geology and zoology in Harvard. Naturally, an attempt was made to apply his views on glaciation to the phenomena of the drift in America. In the summer of 1848, in company with Jules Marcou and a party of students, he undertook the exploration of the Lake Superior region, the results of which were published in 1850. The views set forth relating to the glacial phenomena of the region are of paramount interest.
He argued that the drift of all northeast America and northwest Europe was contemporaneous and due to a general ice sheet. Through a repetition of many of his former arguments, he showed that a current of water sufficiently powerful to transport the large blocks found would have swept practically over the entire globe and not have stopped abruptly, as did the drift, after reaching latitude 39° north. This limit of distribution of the boulders to the northern latitudes also indicated to his mind that the matter of climate was an important factor. Water-transported material, he argued, would not cause straight furrows and scratches, and the theory that such might be due to drifting icebergs was rejected on the ground that existing bergs were insufficient, and to produce such as were would necessitate a period of cold sufficient for his hypothetical polar ice-cap. He pointed out that the northern erratics were rounded and widespread; that the highest hills were scratched and polished to their summits, while to the south the mountain tops had protruded above the ice-sheet and supplied the glaciers with their load of angular boulders. He also called attention to the absence of marine or fresh-water shells from the ground moraine deposits, showing that it was not subaqueous.
Referring to the stratified deposits overlying the drift, he wrote:
The various heights at which these stratified deposits occur above the level of the sea show plainly that since their accumulation the mainland has been lifted above the ocean at different rates in different parts of the country; further, it must be at once obvious that the various kinds of loose material all over the northern hemisphere have been accumulated, not only under different conditions, but during long-continued subsequent periods. To the first, or ice, period belong all phenomena connected with the transportation of erratic boulders, polishing, scratching, etc., during which the land stood at a higher level. To the second period belongs the stratified drift such as indicates a depression of the continent.
In 1856 Dr. Edward Hitchcock came once more to the front, through the medium of the 'Smithsonian Contributions to Knowledge,' with a paper of some 150 royal quarto pages and 12 plates, in which he considered the changes which had taken place in the earth's surface since the close of the Tertiary period. The products of these changes he classed as, first, drift unmodified, and second, drift modified, including under the latter such deposits as ancient and modern beaches, submarine ridges, sea bottoms, osars, dunes, terraces, deltas and moraines. The drift proper he regarded, as before, as a product of several agencies, including icebergs, glaciers, land slips and waves of translation, which, though more active in the past than now, are still in operation.
To account for the drift accumulations at various altitudes he conceived that the water must have stood some 2,500 feet above its present level and, further, that all the northern part of the continent—at least all east of the Mississippi—had been covered by the ocean since the drift period.
As to the origin of the material of the irregular coarse deposit beneath the modified beaches and terraces (ground moraine), he agreed essentially with Naumann in supposing that, first, the eroding materials must have been comminuted stone; second, they must have been borne along under heavy pressure; third, the moving force must have operated slowly and with prodigious energy; and fourth, moving in a nearly uniform direction, though liable to local divergence; fifth, the vehicle of the eroded material could not have been water alone; but, sixth, a firm and heavy mass, somewhat plastic. The exact period of operation of the drift agency he naturally found difficulty in + mining, and felt that, while the greater part of the work was accomplished before the continent had emerged very considerably from the waters, nevertheless, the work of erosion went on for some time after emergence began.
It was in this connection that was made the first suggestion, so far as I am aware, of a possible recurrence of glacial periods, as fully elaborated later by Chamberlin. Eeferring to the occurrence of two series of stria?, the direction of which did not coincide, and the possible existence of still a third series, he wrote: ' Perhaps there were two periods of glaciers, one before, and one subsequent to the drift/
The facts concerning the dispersion of boulders Hitchcock thought could be more satisfactorily explained by icebergs than glaciers, since the transportation and scattering continued until after the time when a large part of the beaches and terraces were formed. Glaciers, he thought, would have plowed tracks through stratified deposits. Icebergs such as now traverse the Atlantic might carry boulders over the beaches and terraces and drop them from time to time, forming thus the intermixture of coarse angular blocks and beach and terrace material, as we now find it.
The supposition that a glacier once existed on this continent wide enough to reach from Newfoundland to the Rocky Mountains is the grand difficulty in the way of the glacial theory.
The writings of the Eogers brothers are singularly lacking in more than casual references to the drift, though, in one case, at least (that relating to the Richmond boulder train) they advanced some theories which were extraordinary, to say the least. In the, for its time, magnificent publication of the first geological survey report of Pennsylvania one would naturally look for an extension of the views of Professor H. D. Eogers, but such, nevertheless, are not found. The fact that he considered, if not fully comprehended, Agassiz's views is shown only by a brief paragraph in which he described and figured drift striæ seen on an exposed surface of umbral sandstone on the south side of the Wyoming Valley. These he described as ' pointing up the slope toward the southwest, as if produced by fragmentary debris violently propelled against the slope of the mountain wall of the valley from the south.' The presence of such ascending striae, both here and elsewhere, effectually refuted, according to his conception of it, the glacial theory of their origin.
Like Hitchcock, he failed to conceive of other than local mountain glaciers of the Swiss type, and he gave the following, even then antiquated, matter for a general discussion of the distribution of the drift and the various phenomena accompanying it. Of the earlier drift, it should be noted, he offered no explanation whatever, other than that implied in a reference to a period of repose 'which separated the convulsed epochs of the earlier general and later local drift.'
A ready explanation of the origin of this newest Pleistocene deposit (i. e., that of the Hudson and Lake Champlain districts) suggests itself when we consider the nature and energy of the crustal movements which lifted the Laurentian clays and sands to a height, in one locality at least, of not less than five hundred feet, and which drained wide tracts of the upper Laurentian Lakes.
The mere agitation or pulsating movement of the crust, if accompanied by any permanent uplift of the land, would suffice, we would think, by lashing the waters of the tidal estuaries in one quarter and the lakes in the other, to strew a portion of the older drift bordering all those basins in wide dispersion upon the top of the more tranquil sediments. But if such a pulsation of the crust were accompanied by successively paroxysmal liftings of wide tracts of the land, then the inundation would take the form of stupendous currents, the strewing power of which would be adequate to any amount of superficial transportation, even to the remote transportation of the larger erratics.
In 1861 the Natural History Survey of Maine was inaugurated and C. H. Hitchcock was placed in charge of that portion relating to geology. Of his work, only that relating to glaciers concerns us here. He noted that the fossiliferous marine clays which were regarded as of the same age as similar deposits along the St. Lawrence and Champlain valleys and referable to the terrace period, sometimes underlay a coarse deposit referable to the modified drift. Without committing himself definitely on this point, he suggested the possibility, therefore, of a recurrence of the drift agencies, that is, a period of second drift, as had the elder Hitchcock fifteen years earlier.
The drift period itself, according to Hitchcock's view as here expressed, was inaugurated by a depression of this portion of the continent amounting to at least 5,000 feet below that of to-day, and it was during this period of depression and reelevation that the drift deposits were formed through the joint agency of icebergs and glaciers.
In 1862 J. S. Newberry expressed his views on glaciers in an article on the 'Surface Geology of the Basins of the Great Lakes.' After reviewing the surface conditions as he saw them, he came to the conclusion that, at a period corresponding in climate, if not in time, with the glacial epoch of the old world, the lake region, in common with all the northern portion of the American continent, was raised several thousand feet above the level of the sea. This was to him the glacial period, during which the surface of the country was planed down and the deep fiords along the Atlantic coast formed. This was followed by a period -of depression, when all the basin of the Great Lakes was flooded with fresh water, forming a vast inland sea in which the laminated blue clays (the oldest drift deposits) were precipitated.
Subsequent to this deposit of blue clay an immense quantity of gravel and boulders was transported from the region north of the Great Lakes and scattered over a wide area south of them. This he regarded as due to floating ice and icebergs.
It would seem that, if one were looking for original observations on drift phenomena, he might turn with safety to the writings of the Canadian geologists. Singularly enough, the views are, if anything, less original than those of the workers on this side of the line. The establishment of a geological survey of Canada under Logan led to the publication of the now well-known volume of 1863. The views here expressed may be accepted as a summary of the knowledge relating to the glaciation on Canadian territory, as it then existed.
Concerning the region of the lake basins of western Canada, Logan wrote:
These great lake basins are depressions, not of geological structure, but of denudation, and the grooves of the surface rocks which descend under their waters appear to point to glacial action as one of the great causes which have produced these depressions. This hypothesis points to a glacial period when the whole region was elevated far above its present level and when the Laurentides, the Adirondacks, and the Green Mountains were lofty Alpine ranges covered with perpetual snow from which great frozen rivers or glaciers extended over the plains below, producing by their movements the glacial drift and scooping out the river valleys and the basins of the great lakes.
In his address before the Natural History Society of Montreal in 1864 J. W. Dawson took occasion to combat vigorously these ideas of Logan, and this on the ground that ' it requires a series of suppositions unlikely in themselves and not warranted by facts'; that it seems physically impossible for a sheet of ice to move over an even surface striating it in uniform directions over vast areas; that glaciers could not have transported the large boulders and left them in the positions found, having no source of supply; that the peat deposits, fossils, etc., show that the sea at that period had much the same temperature as the present arctic currents, and that the land was not covered by ice.
In describing the course of the rock stria? he announced that he had no hesitation in asserting that the force which produced those having a westerly direction was from the ocean into the interior against the slope of the St. Lawrence Valley, and as he could not conceive of a glacier moving from the Atlantic up into the interior, he considered this as at once disposing of the glacial theory. He conceived, rather, that a subsidence took place sufficient to convert all the plains of Canada, New York, and New England into seas. This, he felt, would determine the direction of the Arctic current which moved up the slope. He would account for the excavations of the basins of the Great Lakes by supposing the land so far submerged that an Arctic current from the northeast would pour over the Laurentian rocks on the northern side of Lake Superior and Lake Huron, cutting out the softer strata and at the same time transporting the debris in the form of drift to the southwest. Glaciers were not wholly dispensed with, but limited to regions of mountainous elevation.
J. S. Newberry, while director of the geological survey of Ohio (1869-78) had frequent occasion to discuss glacial phenomena, and a review of his opinions may be given in some detail. Beginning with the later Tertiary times, he believed the following sequence of events to have been established.
(a) During Miocene and Pliocene epochs a continent several hundred feet lower than now, the ocean reaching to Louisville and Iowa, with a subtropical climate prevailing over the lake region, the climate of Greenland and Alaska being as mild as that of southern Ohio at present.
(b) A preglacial epoch of gradual continental elevation, which culminated in the glacial epoch, when the climate of Ohio was similar to that of Greenland at present, and glaciers covered a large part of the surface down to the parallel of forty degrees.
(c) This period followed by another interval of continental subsidence characterized by a warmer climate and melting glaciers and by inland fresh-water seas filling the lake basins, in which were deposited the Erie and Champlain clays, sands and boulders.
(d) Another epoch of elevation which is still in progress.
The sheet of clay and boulders which was found directly overlying the polished surface of the rocks over so large a part of the state, now known under the name of till and boulder clay, he described under the general name of glacial drift, while the loose boulders which he found scattered indiscriminately over the surface, frequently resting on the fine stratified clays, were known under the name of iceberg drift.
If, he wrote, we restore in imagination this inland sea, which we have proved once filled the basin of the lakes, gradually displacing the retreating glaciers, we are inevitably led to a time in the history of this region when the southern shore of this sea was formed by the highlands of Ohio, etc., the northern shore a wall of ice resting on the hills of crystalline and trappean rocks about Lake Superior and Lake Huron.
From this ice wall masses must from time to time have been detached, just as they are now detached from the Humboldt glacier, and floated off southward with the current, bearing in their grasp sand, gravel and boulders—whatever composed the beach from which they sailed. Five hundred miles south they grounded upon the southern shore—the highlands of now western New York, Pennsylvania and Ohio, or the shallows of the prairie region of Indiana, Illinois and Iowa. There melting away and depositing their entire loads.
The loess, as one would naturally expect from the foregoing, was looked upon as the finer sediment deposited in the quiet waters of one of these inland seas, to which the icebergs had no access. The lake basins, with the exception of that of Lake Superior, were regarded as excavated by glacial action—thus agreeing with Logan.
The views of Orton, who succeeded Newberry as state geologist, were not widely different, and, briefly expressed, were as follows:
Threefold divisions of glacial time may be considered as demonstrated: (1) An age of general elevation of northern land, accompanied by intense cold and the formation of extensive continental glaciers; (2) a general depression of the land, with the return of a milder climate; (3) a partial reelevation of the land and a partial return of the cold climate, producing local glaciers and icebergs.
E. T. Cox, while state geologist of Indiana, encountered phenomena in every way similar to those described by Newberry and Orton, and it is to be expected that his mode of accounting for the same would be somewhat similar. In his report of the conduct of the survey of that state (1869-79) he announced his acceptation of the general theory of glacial drift, as at that time understood, and conceived that the necessary climatic changes might be due to the relative position of land and water, and, possibly, a change in the course of the Gulf Stream. He could find, however, no evidence of a subsidence of the land to terminate the glacial period, nor could he find in Ohio, Indiana or Illinois anything to militate against the commencement of the glacial period in Tertiary times and its continuation until brought to a close by its own erosive force aided by atmospheric and meteorological conditions. By these combined agencies acting through time the mountain home of the glacier was cut down and a general leveling of the land took place. This suggestion that the glacial epoch worked out its own destruction through a process of leveling, whereby the altitudes which gave it birth were so far reduced that glaciers could no longer exist, is unique and, so far as the present writer is aware, original with Cox.
The organization in 1876 of a state geological survey of Wisconsin afforded Professor Chamberlin and his assistants opportunity for investigation of the drift phenomena of that state, and in the pages of his reports his views are distinctly formulated. He divided the glacial period into: (1) The terrace or fluviatile epoch, (2) Champlain or lacustrine epoch, (3) the second glacial epoch, (4) the interglacial epoch and (5) the first glacial epoch. This formal announcement of the possibility of two distinct periods of glaciation was here made for the first time, although, as before noted, Edward Hitchcock had at an earlier date suggested such a possibility.
Not content with a mere discussion of the glacial phenomena, Chamberlin considered also matters relating to the cause of glacial movement. The law of flowage he announced as being, in his opinion, similar to that of viscous fluids—this in accordance with the observations of Agassiz, Forbes, Tyndall and others. A later study of Greenland glaciers, as is well known, has caused him to change his views on this point.
In the third edition of his work on 'Acadian Geology,' which appeared in 1878, J. W. Dawson returned once more to a vigorous discussion of the problems of the ice age, and to register again his opposition to the views generally held by American geologists. Many of the arguments used closely resembled those of his former papers and may be reviewed here for the last time.
He regarded the phenomena of the boulder clay and drift in eastern America as due to the action of local glaciers, drift ice and the agency of cold northern currents. Against the theory of a universal glacier he again argued on the ground that such suppositions were not warranted by the facts.
The temperate regions of North America could not be covered with a permanent mantle of ice under existing conditions of solar radiation; for, even if the whole were elevated into a tableland, its breadth would secure a sufficient summer heat to melt away the ice except from high mountain peaks.
For the supposition that such immense mountain chains existed and have disappeared, he found no warrant in geology, and for such an 'unexampled astronomical cause of refrigeration 'as the earth's passing into a colder portion of space, he found no evidence in astronomy. He agreed with Lyell in regarding the theory of the varying eccentricity of the earth as expounded by Croll as insufficient; moreover, it seemed to him physically impossible that a sheet of ice, such as that supposed, could move over an uneven surface, striating it in directions uniform over vast areas and often different from the present inclination of the surface.
He was further influenced in his opinion by the work of Hopkins, who showed, apparently, that only the sliding motion of glaciers could polish or erode rock surfaces, and the internal changes in their mass—the result of weight—could have little or no effect. Glaciers, moreover, he argued could not have transported the boulders great distances and lodged them upon the hill tops, and the universal glacier would, moreover, have no gathering ground for its materials. The huge feldspar boulders from the Laurentide Hills, stranded at Montreal Mountain at a height of six hundred feet above the sea and from fifty to sixty miles further southwest, and which must have come from little, if any, greater elevation and from a direction nearly at right angles to that of the glacial striæ, were against the ice-sheet theory, as were also the large boulders scattered through the marine stratified clays and sands, and the occurrence of marine fossils in the lower part of the drift in the true till near Portland, Maine, and at various points on the St. Lawrence in Canada.
To substantially these views Dawson held to the very last. In his 'Ice Age in Canada' (1893) he is found still combating vigorously the idea that all northern Europe and America were covered by a mer, de glace moving to the southward and outward to the sea, and which moved not only stones and clay to immense distances, but glaciated and striated the whole surface. The glacial theory of Agassiz and others he described as having grown, until, like the imaginary glaciers themselves, it overspread the whole earth. He adopted, rather, what he called the moderate view of Sir Roderick Murchison and Sir Charles Lyell to the effect that Pleistocene subsidence and refrigeration produced a state of our continents in which the lower levels, and at certain periods even the tops of the higher hills, were submerged under water filled every season with heavy field ice formed on the surface of the sea, as at present in Smith's Sound, and also with abundant icebergs derived from glaciers descending from unsubmerged mountain districts. The later Pliocene, so far as Canada was concerned, he considered to be a period of continental elevation and probably of temperate climate.
Thus far the discussion relating to the ice period has been limited wholly to workers and areas east of the Mississippi River. In 1880 and 1882 J. D. Whitney, one time state geologist of California, issued his well-known work on climatic changes of later geological time, in which he discussed the occurrence of glaciers and their possible origin in the west, particularly in the region of the Great Basin. Whitney thought to be able to trace a period of warmth and heavy precipitation, followed by one of desiccation, but anticipated by one of cold and glaciation, the glaciers, however, being limited to the most elevated ranges of the Cordilleras. At the outset he announced himself as opposed to the 'wild and absurd ideas' that had prevailed regarding glaciation in the Sierras, and stated it as his belief that here, at least, ice had played but an extremely subordinate part as a glacial agent, though 'there is no doubt but that the great California range was once covered with grand glaciers, but little if at all inferior to those which now lend such a charm to the Swiss Alps.'
It was Whitney's opinion, further, that the geological importance of the ice sheet had been greatly exaggerated. It seemed to him beyond question that icebergs had played an important part in carrying and distributing the large angular boulders which in many places rest upon the surface in such a manner as to show that they could not have been placed in their present position by running water or by a general ice sheet.
He regarded it as evident enough that the climate of northeastern America during the glacial epoch was a period of greater precipitation than now, but that it was a period of intense cold he would not admit. Glaciation or a glacial period was due merely to increased precipitation. In order that such precipitation should take place, an increased evaporation from the land and water was necessary. This could be brought about only by a general increase of temperature. The amount of precipitation being sufficient, the production of glaciers would depend upon temperature, which itself would be dependent upon local conditions, which again might, or might not, be due to elevation of land surfaces. His idea, in brief, was that while during the glacial epoch there might be over the entire globe a period of sufficient warmth to produce the desired evaporation, the precipitation would fall as rain or snow, according to the local uplift or depression. That the glaciers are now retreating in nearly every instance, he regarded as due, not so much to a change in climate, at least not to a gradual increase of temperature, but rather to a gradual decrease in the amount of annual precipitation.
In this connection, it may be mentioned that Whitney considered the movement of glacial ice due to water:
Glacier ice is not simply ice, but a mixture of ice and water, and it is to the presence of the latter that the whole mass owes its flexibility. The larger the amount of water, other things being equal, the more easily the glacial mass moves. When the water increases so as to get the upper hand, the ice gives way with a rush and becomes an avalanche. . . . The extreme variation of the rate of motion of different glaciers coming down from the inland ice of Greenland is due to the different amounts of water which they have imbibed.
More recent observations than those quoted are familiar, and we may well stop here. That, at a period geologically not very remote, a vast sheet of ice and snow, with all the attributes of a modern glacier, or series of glaciers, covered the northeastern United States and eastern Canada, that this sheet advanced, retreated, and again advanced, and finally utterly disappeared, is the commonly, though not universally, accepted view. The causes which led up to this condition are still problematical. Whether due to cold from increased elevation, as taught by Dana, to astronomical causes, as taught by Croll, or merely to an increase in precipitation, as argued by Whitney, or to a combination of any or all of these causes, is the great problem awaiting solution, if solution is possible on other than a theoretical basis. Chamberlin, the Dawsons, Gilbert, Hall, the Hitchcocks, Lewis, Mather, Newberry, Salisbury, Upham, Winchell, Wright and a score of others have made us acquainted with the physical characteristics of drift deposits and their geographic distribution, but the first-named alone, among Americans, has put forward a satisfactory working hypothesis as to the cause of glacial motion.
Leaving out of consideration Peter Dobson, whose views were not pushed to their legitimate conclusions, the world at large must credit Louis Agassiz, born in Switzerland, but an American by adoption, with being the great promoter and, perhaps, originator of the glacial hypothesis as it exists to-day. His method of procedure, it is interesting to note, consisted in applying what one of our prominent geologists has slightingly referred to as the principle of prolonging the harmless and undestructive rate of geological change of to-day backwards into the deep past.