CHAPTER III.
CHRONOLOGICAL SUCCESSION OF THE FOSSILIFEROUS ROCKS.
The physical geologist, who deals with rocks simply as rocks, and who does not necessarily trouble himself about what fossils they may contain, finds that the stratified deposits which form so large a portion of the visible part of the earth's crust are not promiscuously heaped together, but that they have a certain definite arrangement.

In each country that he examines, he finds that certain groups of strata lie above certain other groups; and in comparing different countries with one another, he finds that, in the main, the same groups of rocks are always found in the same relative position to each other.

It is possible, therefore, for the physical geologist to arrange the known stratified rocks into a successive series of groups, or "formations," having a certain definite order.

The establishment of this physical order amongst the rocks introduces, however, at once the element of _time_, and the physical succession of the strata can be converted directly into a historical or _chronological_ succession.

This is obvious, when we reflect that any bed or set of beds of sedimentary origin is clearly and necessarily younger than all the strata upon which it rests, and older than all those by which it is surmounted.
It is possible, then, by an appeal to the rocks alone, to determine in each country the general physical succession of the strata, and this "stratigraphical" arrangement, when once determined, gives us the _relative_ ages of the successive groups.