Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/150

110 Primary, Secondary, and Tertiary Ages, with their characteristic fossils, according to many Geologists, were not confined to America, but extended all over the globe; the whole earth having passed at the same time successively through the Primary, Secondary, and Tertiary Ages. Some few Geologists do not accept this onion-coat hypothesis, which supposes that similar rocks, with similar remains, were deposited at the same time all round the earth, like the layers in the coat of an onion. With all deference to Geologists, let us examine the tests used for determining the time of the deposition of foreign rocks as compared with our own. The test of having similar minerals, when applied to, elucidating the age of foreign rocks as compared with those on this continent, is worthless, since the chalk, sandstone, etc. of which the rocks are composed are forming in all ages; while a determination of the age of rocks, based on the order in which they follow or overlie one another,—when applied, for example, to New York and England, separated by an ocean,—to say the least, is very unreliable. The third test, that rocks having similar organic remains are of the same age, considered by most Geologists as settling the question, whenever such comparisons are possible, may be as fruitful a source of error as the view that similar minerals deposited in the same way are of the same age. Nor does the reverse of this proposition hold good, that rocks are of a different age because they contain different fossils. Suppose, for example, that the western part of North America and Australia were gradually to sink into the sea, as parts of the world are now doing, and then slowly to rise again, the Geologist of an indefinitely remote future might argue, because he found many fossil pouch-bearing animals in Australia, and the bones of an extinct human race in America, that the Kangaroo was not contemporaneous with the Indian. From the distribution of plants and animals at the present time, we know that