Darwinism and the evolution of man / by H.C. March.

  • March, Henry Colley.
Date:
1883
Catalogue details

Licence: Public Domain Mark

Credit: Darwinism and the evolution of man / by H.C. March. Source: Wellcome Collection.

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    have, in their turn died, and their hard coverings have been slowly pierced by other creatures, while yet the deposit had not covered the mollusc to half its thickness. Not less slow is denudation. In faults, when one side of a dislocation has been raised perhaps thousands of feet above the other, and both have been washed down to a level again, though it can be known how much more has been denuded on one side than on the other to restore the level, it cannot be known how much has also been taken from both sides. A fault crossing from Dunbar to the Ayrshire coast, amounts to nearly three miles vertical. This dislocation has on the south side the ancient silurian rocks on which rest the carboniferous. On the north, between the carboni- ferous and the silurian occurs the old red sandstone. So that the dislocation is older than the carboniferous era. For originally the silurian rocks, south of the fault, must have been covered by the prolongation of the old red sandstone, afterwards completely removed by denudation. Thus the enormous thickness of three miles of old red sandstone must have been denuded away during the period which intervened between its deposition and the subsequent accumulation of the carboniferous limestone and coal- measures now lying directly upon the silurian rocks. The quantity of sediment brought down by the Mississippi has been carefully investigated by order of the United States Government, The result is that the mean level of its enormous area of drainage must be lowered 4566th of a foot annually, or one foot in 4566 years. Mr. Croll, Mr. Geikie,and Sir Charles Lyell consider that denudation takes place at the rate of one foot in 6000 years. Take double this rate, and at two feet in 6000 years the time occupied in denuding three miles of old red sandstone would be 45 millions of years, and this takes no account of the time occupied in deposition. But older than the old red sandstones, and before they were deposited, the silurian formations had been denuded to depths of thousands of feet, and these ancient formations were themselves deposited in the ocean by the slow denudation of the Cambrian rocks. These, in turn, had been formed from the earlier Lauren- tian strata; and the Laurentian rocks themselves were built up from the ruins of other rocks, which were themselves sedimentary rocks, and not the primary rocks of the globe. On the other hand, above the carboniferous, come the Permian, Triassic, Oolitic, Cretaceous, Tertiary, and recent rocks ; the whole amounting in thickness to nearly 14 miles of British strata. Surely then, sufficiency of time is less a postulate than a demonstrable fact; and we are at last fully entitled to claim that the truth of Darwinism is abundantly proved. In other words, the evidence in its favour compels belief. But there are two classes of persons whom no amount of evidence will convince. Those who are deeply pledged and