388 PEINCIPLES AND METHODS OF PALAEONTOLOGY. 



even assuming them to have been deposited, without interruption, at a 

 rate faster than any sea or lake deposits mud now-a-days, we slioidd 

 still require a period six times as long as that of which any human 

 record exists, for their formatiou. But, in truth, when we take into 

 account the probably immensely greater time required for the formation 

 of two feet of sedimentary deposit, the vast amount of rock which has 

 been formed and subsequently swept away hy denudation, so that it is 

 not reckoned in estimating- this total thickness of the strata, and the 

 possibility that nmsses of strata, which will requii*e interpolation in the 

 general series, lie hidden from our view in parts of the world which 

 have not yet been examined, or under the present bed of the sea, the 

 most sober calculator will hardly venture to limit the factor by which 

 even a period of thirty thousand years should be multiplied to give the 

 whole period recorded by the monuments of geology. 



The conclusions here drawn from the facts of physical geology are in 

 perfect unison with the chronological indications afforded by fossils. 

 Beds many feet in thickness, composed of the remains of marine ani- 

 mals, their shells unbroken and uudistnrbed, and sometimes covered 

 with parasitic growths, (just like recent dead shells which remain long 

 undisturbed at the bottom of the ocean,) are constantly met with. Here 

 and there are thick sti-ata, composed of nothing but the remains ot 

 microscopic plants and animals, which must have required a vast time 

 for their aggregation ; elsewhere, the vestiges of huge coral reefs testify 

 that innumerable generations of their slowly-growing fabricators must 

 have lived and died undisturbed, in one locality; and, in some places, 

 enorntous accumulations of the bones of large vertebrata, each individ- 

 ual of which must have required many years to attain its full growth, 

 tell the same tale. 



The two great astronomical truths to which the general mind has 

 always found the greatest difficulty in assenting are, first, the doctrine 

 that the seemingly fixed earth moves, while the apparently moving sun 

 stands still ; secondly, that the earth is but a i^article, and the diameter 

 of the system to which it belongs insignificant when compared with the 

 vast space which separates one of the greater heavenly bodies from 

 another. Geology presents two corresjionding truths, as hard to believe 

 and yet as well founded. The first is, that the seemingly fixed land is 

 subject to incessant oscillations, while the sea, so mobile on the small 

 scale, remains i]i reality comparatively unchanged. The other is, that 

 our historical period, even if we include the widest limit to which tra- 

 dition woidd carry the records of our race, is but an insignificant portion 

 of the countless ages which have elapsed since the animals, the remains 

 3f which are exposed to view in the Lower Silurian cases of this collec- 

 tion, lived and died, and were buried in the oozy bed of the ocean of 

 that period. 



We are, therefore, compelled to believe that a general uniformity has 

 prevailed in the operations of physical and vital nature throughout all 

 time of which we haveany record ; but just as the generally uniform and 

 regular movement of the celestial bodies is quite consistent with minor 

 and subordinate xierturbations, so the proved uniformity of action of 

 the causes in operation in the physical world by no means excludes the 

 l)ossibility of occasional sudden and immense changes, or "catastrophes," 

 as they have been called; nor does the equally evident general uniformity 

 of i>lan predominant throughout the ancient fauna and flora in any way 

 interfere with very great and imj^ortant deviations from those which 

 now exist. 



