Page:South African Geology - Schwarz - 1912.djvu/138

 the deposits laid down in the lakes of the Triassic times. The age of a particular bed, therefore, must not be judged from one or two specimens of fossils, but the whole group of animals or fauna that lived when the sediment was laid down must be taken into account. In freshwater deposits it is the collection of the remains of plants, or the flora, which is important. Unfortunately, however, they are much less frequently preserved than animals.

It will be noticed in the Table of Strata that the South African Devonian Rocks are correlated to, or paralleled with, the American, not the European marine, Devonian. This is because the South African fossils correspond to the American and not the European forms. This, then, shows that beds may be contemporary and yet may not contain similar fossils. The reason why this happens is that in this particular case the sea in which the animals lived, whose remains are found in the European rocks, was separated by a vast land barrier from the American and South African sea, which existed at that time, and the animals that lived in the two oceans were different, as is the case now with the animals which live respectively in the Atlantic and Pacific Oceans.

The converse holds good: beds containing similar fossils may not be contemporary. Take as an illustration the cave deposits of the south of France. In any one cave one may find three layers overlying each