Imágenes de página

the surface of the water, had not other forces been subsequently employed to raise them into dry land.

"Wherever solid matter arose 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 early 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 thickness of many miles."—Bridgewater Treatise, pp. 42, 50, 51.

"Thus the origin of strata is derived from depositions of the materials of the dry land under the waters of the sea, and, in some cases, of great inland lakes intermixed with the spoils of animals that have lived and died through a long succession of ages. If the daily causes of waste pulverize the solid mountains, and the rivers transport their ruins to the sea, so other causes acting more extensively and powerfully, must be allowed a share in producing and depositing the materials to which we owe our present stratified rocks. The extent and nature of those operations will be fully examined in its proper place, as they are now in progress, or are past; and as they include the geology which relates to the present surface of the earth. In the ruins of an ancient earth we find the materials which formed the present; as in the destruction of the land which we now inhabit nature seems to be preparing habitations for future races of animated beings.

"But though I have here said that causes operating more extensively and powerfully than the slow actions of waste and transportation may have aided in preparing the materials of the strata, we must beware of allowing more effect than they were capable of producing, as has been done by those who object to certain geological claims on indefinite time, and who seek for solutions in transitory diluvian powers. The effects of such torrents must have been to deposit mixed materials of various sizes in a confused manner ; and they could therefore have prepared the germs of the conglomerated strata only. The strata formed of finer materials must have been the consequences of tedious actions, analogous to those which we daily witness; while their separation into distinct rocks, into alternations of clay and sand, producing schist and sandstone, must have equally been the work of a slow process beneath the water."—Macculloch's Geology, vol. i. pp. 81, 82.

"Denudation is the removal of solid matter by running water, whether by a river or marine current, and the consequent laying bare of some inferior rock. Geologists have, perhaps, been seldom in the habit of reflecting that this operation has exerted an influence on the structure of the earth's crust as universal and important as sedimentary deposition itself; for denudation is the inseparable accompaniment of the production of all new strata of mechanical origin. The formation of every new deposit by the transport of sediment and pebbles necessarily implies that there has been somewhere else a grinding down of rocks into rounded fragments, sand, or mud, equal in quantity to the new strata. All depositions, therefore, except in the case of a shower of volcanic ashes, is the sign of superficial waste going on contemporaneously, and to an equal amount elsewhere. The gain at one point is no more than sufficient to balance the loss at some other.

"If then the entire mass of stratified deposits in the earth's crust is at once the monument and measure of the denudation which has taken place, on how stupendous a scale ought we to find the signs of this removal of transported materials in past ages?

"Professor Ramsay has shown that the missing beds removed from the summit of the Mendips must have been nearly a mile in thickness, and he has pointed out.consider-able areas in South Wales and some of the adjacent counties of England where a series of palaeozoic strata not less than 11,000 feet in thickness have been stripped off. All these materials have of course been transported to new regions, and have entered into the composition of more modern formations. On the other hand, it is shown by observations in the same 'Survey,' that the palaeozoic strata are from 20,000 to 30,000 feet thick. It is clear that such rocks, formed of mud and sand, now for the most part consolidated, are the monuments of denuding operations, which took place at a very remote period in the earth's history." —LyeWs Manual, pp. 66-68.

"The strata are accumulations of consolidated sand and other detritus, the sedimentary deposits of rivers and seas, combined with the durable remains of animals and plants.

"From the first moment that dry land appeared on the earth's surface, whatever may have been the material of which it was composed, the disintegrating effects of atmospheric agents and of water in motion must have commenced. The detritus thus produced transported to the tranquil depths of the ocean, would then subside in successive layers, and a series of sedimentary strata be gradually formed; and after the creation of living things, the durable remains of animals and vegetables must have become intermingled with the detritus of the land, and imbedded in the deposits then in progress.

"If the land were sterile, destitute of vegetation, and untenanted by any species of animals, the relics of the inhabitants of the sea would alone be imbedded; on the contrary, if the sediments were produced by the action of streams and rivers flowing through a country covered with forests, and swarming with animal life, the strata accumulated in lakes and inland bays would teem with the remains of terrestrial and fluviatile animals and plants."—MantelVs Geological Principles, in his Excursion round the Isle of Wight, pp. 56-58.

"It is universally admitted that the materials of the sedimentary strata . . . are derived from the disintegration, decomposition, and abrasion of older rocks, and from animal and vegetable secretions."—Mather's Geology of the first Geological District of New York, p. 213.

They thus universally exhibit the strata as formed from detritus borne down by streams and rivers from pre-existing continents and islands and distributed over the bed of the sea.

The surface of that imagined primitive earth, instead of loose soils and strata that are easily disintegrated and borne by torrents and rivers to the sea, consisted, according to these writers, exclusively of granite, one of the most solid and indestructible of the rocks.

"Assuming that the whole materials of the globe may have once been in a fluid or even a nebular state, 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 crystallization 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."—Buckland's Bridgewater Treatise, p. 40.

"That granite has in reality furnished a very large part

« AnteriorContinuar »