The chief reason why I wished you to understand this is, that you may see how it is known that one stratum is older than another. It is evident that the substance marked a, in the section, which is limestone, must have been deposited before b, while b must certainly be older than c.
Now in most of the strata above the granite, which is nearly always in the position of the oldest formation, there are found various shells, plants, and bones of animals; and where certain remains of different animals or vegetables are found in one stratum, it is concluded that they must have been living about the same time.
Most of the animals of the older strata were different in form from any at present known to exist; and some of them are very remarkable, and if they were alive now, would seem to us very strange and awkward.
EXTINCT ANIMALS.