Geology and Mineralogy considered with reference to Natural Theology/Chapter 4

From Wikisource
Jump to: navigation, search


Relation of Unstratified to Stratified Rocks.

I shall enter into no further details respecting the component members of each group of stratified rocks, than are represented by the lines of division and colours upon the section.[1] They are arranged under the old divisions of primary, transition, secondary, and tertiary series, more from a sense of the convenience of this long received arrangement, than from the reality of any strongly defined boundaries by which the strata, on the confines of each series, are separated from one another.

As the materials of stratified rocks are in great degree derived, directly or indirectly, from those which are unstratified,[2] it will be premature to enter upon the consideration of derivative strata, until we have considered briefly the history of the primitive formations. We therefore commence our inquiry at that most ancient period, when there is much evidence to render it probable that the entire materials of the globe were in a fluid state, and that the cause of this fluidity was heats The form of the earth being that of an oblate spheroid, compressed at the poles, and enlarged at the equator, is that which a fluid mass would assume from revolution round its axis. The further fact, that the shortest diameter coincides with the existing axis of rotation, shows that this axis has been the same ever since the crust of the earth attained its present solid form.

Assuming that the whole materials of the globe may have once been in a fluid, or even a nebular state,[3] from the presence of intense heat, the passage of the first consolidated portions of this fluid, or nebulous matter, to a solid state may have been produced by the radiation of, heat from its surface into space; the gradual abstraction of such heat would allow the particles of matter to approximate and crystallize; and the first result of this crystal ligation might have been the formation of a shell or crust, composed of oxidated metals and metalloids, constituting various rocks of the granitic series, around an incandescent nucleus, of melted matter, heavier than granite; such as forms the more weighty substance of basalt and compact lava.

It is now unnecessary to dwell on controversies which have prevailed during the last half century, respecting the origin of this large and important class of unstratified crystalline rocks, which the common consent of nearly all modern geologists and chemists refers to the action of fire. The agency of central heat, and the admission of water to the metalloid bases of the earths and alkalis, offer two causes which, taken singly or conjointly, seem to explain the production and state of the mineral ingredients of these rocks; and to account for many of the grand mechanical movements that have affected the crust of the globe. The gradations are innumerable, wlflch connect the infinite varieties of granite, syenite, porphyry, greenstone, and basalt with the trachytic porphyries and lavas that are at this day ejected by volcanos. Although there still remain some difficulties to be explained, there is little doubt that the fluid condition in which all unstratified crystalline rocks originally existed, was owing to the solvent power of heat; a power whose effect in melting the most solid materials of the earth we witness in the fusion of the hardest metals, and of the flinty materials of glass.[4]

Beneath the whole series of stratified rocks that appear on the surface of the globe (see section Pl. 1), there probably exists a foundation of unstratified rocks; bearing an irregular surface, from the detritus of which the materials of stratified rocks have in great measure been derived,[5] amounting, as we have stated, to a thickness of many miles. This is indeed but-a small depth, in comparison with the diameter of the globe; but small as it is, it affords certain evidence of a long series of changes and revolutions; affecting not only the mineral condition of the nascent surface of the earth, but attended also by important alterations in animal and vegetable life.

The detritus of the first dry lands, being drifted into the sea, and there spread out into extensive beds of mud an sand and gravel, would for ever have remained beneath the surface of the water, had not other forces been subsequently employed to raise them into dry land: these forces appear to have been the same expansive powers of heat and vapour which, having caused the elevation of the first raised portions of the fundamental crystalline rocks, continued their energies through all succeeding geological epochs, and still exert them in producing the phenomena of active volcanos; phenomena incomparably the most violent that now appear upon the surface of our planet.[6]

The evidence of design in the employment of forces, which have thus effected a grand general purpose, viz. that of forming dry land, by elevating strata from beneath the waters in which they were deposited, stands independent of the truth or error of contending theories, respecting the origin of that most ancient class of stratified rocks, which are destitute of organic remains (see Pl. 1.—section 1, 2, 3, 4, 5, 6, 7). It is immaterial to the present question, whether they were formed (according to the theory of Hutton) from the detritus of the earlier granitic rocks, spread forth by water into beds of clay and sand; and subsequently modified by heat: or whether they have been produced, (as was maintained by Werner) by chemical precipitation from a fluid, having other powers of solution than those possessed by the waters of the present ocean. It is of little importance to our present purpose, whether the non-appearance of animals and vegetables in these most ancient strata was caused by the high temperature of the waters of the ocean, in which they are mechanically deposited; or by the compound nature and uninhabitable condition of a primeval fluid, holding their materials in solution. All observers admit that the strata were formed beneath the water, and have been subsequently converted into dry land: and whatever may have been the agents that caused the movements of the gross unorganized materials of the globe; we find sufficient evidence of prospective wisdom and design, in the benefits resulting from these obscure and distant revolutions, to future races of terrestrial creatures, and more especially to Man.[7]

In unstratified crystalline rocks, wholly destitute of animal or vegetable remains, we search in vain for those most obvious evidences of contrivance, which commence with the first traces of organic life, in strata of the transition period; the chief agencies which these rocks indicate, are those of fire and water; and yet even here we find proof of system and intention, in the purpose which they have accomplished, of supplying and accumulating at the bottom of the water the materials of stratified formations, which, in after times, were to be elevated into dry lands, in an ameliorated condition of fertility. Still more decisive are the evidences of design and method, which arise from the consideration of the structure and composition of their crystalline mineral ingredients. In every particle of matter to which crystallization has been applied, we recognize the action of those undeviating laws of polar forces, and chemical affinity, which have given to all crystallized bodies a series of fixt definite forms and definite compositions. Such universal prevalence of law, method, and order assuredly attests the agency of some presiding and controlling mind. A further argument, which will be more insisted on in speaking on the subject of metallic veins, may be founded on the dispensation whereby the primary and transition rocks are made the principal repositories of many valuable metals, which are of such peculiar and indispensable importance to mankind.

  1. For particular information respecting the mineral character and organic remains of the strata composing each series, I must refer to the numerous publications that have been devoted to these subjects. A most convenient summary of the contents of these publications will be found in De La Beche's Manual of Geology, and in Von Meyer's Palæologia, (Frankfurt, 1832;) ample details respecting the English strata are given in Conybeare and Phillips's Geology of England and Wales. See also Bakewell's Introduction to Geology, 1833; and Professor Phillips's article Geology, in the Encyclopædia Metropolitana; also Professor Phillips's Guide to Geology, 8vo. 1834; and De La Beche's Researches in Theoretical Geology, 8vo. 1834. The history of the organic remains of the tertiary period has been most ably elucidated in Lyell's Principles of Geology.
  2. In speaking of crystalline rocks of supposed igneous origin as unstratified, we adopts distribution which, though not strictly accurate, has long been in general use among geologists. Ejected masses of granite, basalt, and lava have frequently horizontal partings, dividing them into beds of various extent and thickness, such as those which are most remarkable in what the Wernerians have called the Floetz trap formation, Pl. 1, section Fig. 6.; but they do not present that subdivision into successions of small beds, and still smaller laminæ, which usually exists in sedimentary strata that have been deposited by the action of water.
  3. The nebular hypothesis offers the most simple, and therefore the most probable theory, respecting the first condition of the material elements that compose our solar system. Mr. Whewell has shown how far this theory, supposing it to be established, would tend to exalt our conviction of the prior existence of some presiding Intelligence.—Bridgewater Treatises, No. III. Chap. vii.
  4. The experiments of Mr. Gregory Watt on bodies cooled slowly after fusion; and of Sir James Hall, on reproducing artificial crystalline rocks, from the pounded ingredients of the same rocks highly heated under strong pressure: and the more recent experiments of Professor Mitscherlich, on the production of artificial crystals, by fusion of definite proportions of their component elements, have removed many of the objections, which were onk urged against the igneous origin of crystalline rocks.

    Professor Kersten has found distinctly formed crystals of prismatic Felspar on the walls of a furnace in which Copper slate and Copper Ores had been melted. Among these pyrochemically formed crystals, some were simple, others twin. They are composed of Silica, Alumina, and Potash. This discovery is very important, in a geological point of view, from its bearing' on the theory of the igneous origin of crystalline rocks, in which Felspar is usually so large an ingredient. Hitherto every attempt to make felspar crystals by artificial means has failed. See Poggendorf's Annalen, No. 22, 1834, and Jameson's Edin. New. Phil. Journal.

  5. Either directly, by the accumulation of the ingredients of disintegrated granitic rocks; or indirectly, by the repeated destruction of different classes of stratified rocks, the materials of which had, by prior operations, been derived from unstratified formations.
  6. "The fact of great and frequent alteration in the relative level of the sea and land is so well established, that the only remaining questions regard the mode in which these alterations have been effected, whether by elevation of the land itself§ 'or subsidence in the level of the sea? And the nature of the force which has produced them? The evidence in proof of great and frequent movements of the land itself, both by protrusion and subsidence, and of the connexion of these movements with the operations of volcanos, is so various and so strong, derived from so many different quarters on the surface of the globe, and every day so much extended by recent inquiry, as almost to demonstrate that these have been the causes by which those great revolutions were effected; and that although the action of the inward forces which protrude the land has varied greatly in different countries, and at different periods, they are now and ever have been incessantly at work in operating present change and preparing the way for future alteration in the exterior of the globe."—Geological sketch of the Vicinity of Hastings, by Dr. Fitton, pp. 85, 86.
  7. In describing geological phenomena, it is impossible to avoid the use of theoretical terms, and the provisional adoption of many theoretical opinions as to the manner in which these phenomena have been produced. From among the various and conflicting theories that have been proposed to explain the most difficult and complicated problems of Geology. I select those which appear to carry with them the highest degree of probability; but as results remain the same from whatever cause they have originated, the force of inferences from these results will be unaffected by changes that may arise in our opinions as to the physical causes by which these have been produced. As in estimating the merits of the highest productions of human art it is not requisite to understand perfectly the nature of the machinery by which the work has been effected in order to appreciate the skill and talent of the artist by whom it was contrived; so our minds may be fully impressed with a perception of the magnificent results of creative intelligence, which are visible in the phenomena of nature, although we can but partially comprehend the mechanism that has been instrumental to their production; and although the full development of the workings of the material instruments by which they were effected, has not yet been, and perhaps may never be, vouchsafed to the prying curiosity of man.