The task, however, of the physical geologist in this matter is immensely lightened when he calls in palaeontology to his aid, and studies the evidence of the fossils embedded in the rocks.

Not only is it thus much easier to determine the order of succession of the strata in any given region, but it becomes now for the first time possible to compare, with certainty and precision, the order of succession in one region with that which exists in other regions far distant.

The value of fossils as tests of the relative ages of the sedimentary rocks depends on the fact that they are not indefinitely or promiscuously scattered through the crust of the earth,--as it is conceivable that they might be.

On the contrary, the first and most firmly established law of Palaeontology is, that _particular kinds of fossils are confined to particular rocks_, and _particular groups of fossils are confined to particular groups of rocks_.

Fossils, then, are distinctive of the rocks in which they are found--much more distinctive, in fact, than the mere mineral character of the rock can be, for _that_ commonly changes as a formation is traced from one region to another, whilst the fossils remain unaltered.