. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Ch. IX,] OF AQUEOUS ROCKS. 97 leagues in length, so as to pass from the tropics into the temperate zone. If the bottom of the sea be afterwards upraised, the organic remains imbedded in such yellow or red strata may indicate the different animals or plants which once inhabited at the same time the temperate and equatorial regions. It may be true, as a general rule, that groups of the same species of animals and plant's may extend over wider areas than deposits of hom
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Ch. IX,] OF AQUEOUS ROCKS. 97 leagues in length, so as to pass from the tropics into the temperate zone. If the bottom of the sea be afterwards upraised, the organic remains imbedded in such yellow or red strata may indicate the different animals or plants which once inhabited at the same time the temperate and equatorial regions. It may be true, as a general rule, that groups of the same species of animals and plant's may extend over wider areas than deposits of homo- geneous composition; and if so, palseontological characters will be of more importance in geological classification than the test of mineral com- position ; but it is idle to discuss the relative 7alue of these tests, as the aid of both is indispensable, and it fortunately happens, that where the one criterion fails, we can often avail ourselves of the other. Test hj included fragments of older rocks.—It was stated, that inde- pendent proof may sometimes be obtained of the relative date of two formations, by fragments of an older rock being included in a newer one. This evidence may sometimes be of great use, where a geologist is at a loss to determine the relative age of two formations from want of clear sections exhibiting their true order of position, or because the strata of each group are vertical. In such cases we sometimes discover that the more modern rock has been in part derived from the degradation of the older. Thus, for example, we may find chalk with flints in one part of a country; and, in another, a distinct formation, consisting of alternations of clay, sand, and pebbles. If some of these pebbles consist of similar flint, including fossil shells, sponges, and forarniniferee, of the same species as those in the chalk, we may confidently infer that the chalk is the oldest of the two formations. Chronological groups.—The number of groups into which the foss
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
