Imágenes de página

ble planet, through long successions of change and of convulsive movements, to a tranquil state of equilibrium; in which it has become the convenient and delightful habitation of man, and of the multitudes of terrestrial creatures that are his fellow tenants of its actual surface.*

Primary Stratified Rocks.

In the summary we have given of the leading phenomena of unstratified and volcanic rocks, we have unavoidably been led into theoretical speculations, and have seen that the most probable explanation of these phenomena is found in the hypothesis of the original fluidity of the entire materials of the earth, caused by the presence of intense heat. From this fluid mass of metals, and metalloid bases of the earths, and alkalies, the first granitic crust appears to have been formed, by oxidation of these bases; and subsequently broken into fragments, disposed at unequal levels above and below the surface of the first formed seas.

Wherever sobd matter rose above the water, it became exposed to destruction by atmospheric agents; by rains, torrents, and inundations; at that time probably acting with intense violence, and washing down and spreading forth, in the form of mud and sand and gravel, upon the bottom of the then existing seas, the materials of primary stratified rocks, which by subsequent exposure to various degrees of subterranean heat, became converted into beds of gneiss, and mica slate, and hornblende slate, and clay slate. In the detritus thus swept from the earliest lands into the most ancient seas, we view the commencement of that enormous series of derivative strata which, by long continued repetition of similar processes, have been accumulated to a thick ness of many miles.*

* See farther details respecting the effects of volcanic forces in the description of PI . I. Vol. ii.

* Mr. Conybeare (in his admirable Report on Geology to the British Association for the advancement of Science, 1832, p. 367) shows, that many of the most important principles of the igneous theory, which has been almost demonstrated by modern discoveries, had been anticipated by the universal Leibnitz "In the fourth section of his Protogsa, Leibnitz presents us with a masterly sketch of his general views, and, perhaps, even in the present day, it would be difficult to lay down more clearly the fundamental positions which must be necessarily common to every theory, attributing geological phenomena in great measure to central igneous agency. He attributes the primary and fundamental rocks to the refrigeration of the crust of this volcanic nucleus; an assumption which well accords with the now almost universally admitted igneous origin of the fundamental granite, and with the structure of the primitive slates, for the insensible gradation of these formations appears to prove that gneiss must have undergone in a greater, and mica slate in a less degree the same action of which the maii mum intensity produced granite.

"The dislocations and deranged position of the strata he attributes to the breaking in of vast vaults, which the vesicular and cavernous structure assumed by masses, during their refrigeration from a state of fusion must necessarily have occasioned in the crust, thus cooling down and consolidated. He assigns the weight of the materials and the eruption of elastic vapours as the concurrent causes of these disruptions; to which we should perhaps add, that the oscillations of the surface of the still fluid nucleus may, independently of any such cavities, have readily shattered into fragments the refrigerated portion of the crust; especially, as at this early period, it must have been necessarily very thin, and resembling chiefly the scoria e floating on a surface of lava just beginning to cool. He justly adds, that these disruptions of the crust must, from the disturbances communicated to the incumbent waters, have been necessarily attended with diluvial action on the largest scale. When these waters had subsequently, in the intervals of quiescence between these convulsions, deposited the materials first acquired by their force of attrition, these sediments formed, by their consolidation, various stony and earthy strata. Thus, he observes, we may recognise a double origin of the rocky masses, the one by refrigeration from igneous fusion, (which, as we have seen, he considered principally to be as

VOL. I.—5 t

The total absence of organic remains throughout those lowest portions of these strata, which have been called primary, is a fact consistent with the hypothesis which forms part of the theory of gradual refrigeration; viz. that the waters of the first formed oceans were too much heated to have been habitable by any kind of organic beings.*

In these most ancient conditions, both of land and water, Geology refers us to a state of things incompatible with the existence of animal and vegetable life; and thus on the evidence of natural phenomena, establishes the important fact that we find a starting point, on this side of which all forms, both of animal and vegetable beings, must have had a beginning.

As, in the consideration of other strata, we find abundant evidence in the presence of organic remains, in proof of the exercise of creative power, and wisdom, and goodness, attending the progress of life, through all its stages of advancement upon the surface of the globe; so, from the absence of organic remains in the primary strata, we may derive an important argument, showing that there was a point of time in the history of our planet, (which no other researches but those of geology can possibly approach,) antecedent to the beginning of either animal or vegetable life. This conclusion is the more important, because it has been the refuge of some speculative philosophers to refer the origin of existing organizations, either to an eternal succession of the same species, or to the formation of more recent from more ancient species, by successive developments, without the interposition of direct and repeated acts of creation ; and thus, to deny the existence of any first term, in the infinite series of successions which this hypothesis assumes. Against this theory, no decisive evidence has been accessible, until the modern discoveries of geology had established two conclusions of the highest value in relation to this long disputed question: the first proving, that existing species have had a beginning; and this at a period comparatively recent in the physical history of our globe: the second showing that they were preceded by several other systems of animal and vegetable life, respecting each of which it may no less be proved, that there was a time when their existence had not commenced; and that to these more ancient systems also, the doctrine of eternal succession both retrospective and prospective, is equally inapplicable.*

signable to the primary and fundamental rocks,) the other by concretion from aqueous solution. We have here distinctly slated the great basis of every scientific classification of rock formations. By the repetition of similar causes (i. e. disruption of the crust and consequent inundations) frequent alternations of new strata were produced, until at length these causes having been reduced to a condition of quiescent equilibrium, a more permanent state of things emerged. Have we not here clearly indicated the data on which, what may be termed the chronological investigation of the scries of geological phenomena, must ever proceed?"

* So long as the temperature of the earth continued intensely high, water could have existed only in the state of steam or vapour, floating in the atmosphere around the incandescent surface.

Having this evidence both of the beginning and end of several systems of organic life, each affording internal proof of the repeated exercise of creative design, and wisdom, and power, we are at length conducted back to a period anterior to the earliest of these systems; a period in which we find a series of primary strata, wholly destitute of organic remains; and from this circumstance, we infer their deposition to have preceded the commencement of organic life. Those who contend that life may have existed during the formation of the primary strata, and the animal remains have been obliterated by the effects of heat, on strata nearest to the granite, do but remove to one point farther backwards the first term of the finite series of organic beings; and there still remains beyond this point an antecedent period, in which a state of total fusion pervaded the entire materials of the fundamental granite; and one universal mass of incandescent elements, wholly incompatible with any condition of life, which can be shown to have ever existed, formed, the entire substance of the globe.*

* Mr. Lyell, in the first four chapters of the second volume of his Principles of Geology, has very ably and candidly examined the arguments that .have been advanced in support of the doctrine of transmutation of species, and arrives at the conclusion,—" that species have a real existence in nature, and that each was endowed, at the time of its creation, with the attribute. and organization by which it is now distinguished."

Mr. De la Bcche also says (Geological Researches, 1834, p. 239, 1st. edit Svo.) "There can be no doubt that many plants can adapt themselves to altered conditions, and many animals accommodate themselves to different climates ; but when we view the subject generally, and allow full importance to numerous exceptions, terrestrial plants and animals seem intended to fill the situations they occupy, as these were fitted for them; they appear created as the conditions arose, the latter not causing a modification in previous)* existing forms productive of new species."

• In adopting. the hypothesis that the primary stratified rocks have been altered and indurated by subjacent heat, it should be understood, that although heat is in this case referred to as one cause of the consolidation of strata, there are other causes which have operated largely to consolidate the secondary and tertiary strata, which are placed at a distance above rocks of igneous origin. Although many kinds of limestone may have been in certain cases converted to crystalline marble, by the action of heat under high pressure, there is no need for appealing to such agency to explain the consolidation of ordinary strata of carbonate of lime; beds of secondary and tertiary sandstone have often a calcareous cement, which may have been precipitated from water, like the substance of stalactites and ordinary limestone. When their cement is siliceous, it may also have been supplied by some humid process, analogous to that by which the siliceous matter of chalcedony and of quartz is either suspended or dissolved in nature; a process, the existence of which we cannot deny, although it has yet baffled all the art of chemistry to imitate it. The beds of clay which alternate with limestone, and sand, or sandstone, in secondary and tertiary formations, show no indications of the action of heat; having undergone no greater consolidation than may be referred to pressure, or to the admixture of certain proportions of carbonate of lime, where the clay beds pass into marl and marlstone. Beds of soft unconsolidated clay, or of loose unconsolidated sand, are very rarely if ever found amongst any of the primary strata, or in the lower regions of the transition formation; the effects of heat appear to have converted

« AnteriorContinuar »