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. The task, however, of the physical geologist in this matter is immensely lightened when he calls in palæontology 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 Palæontology 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. It would therefore be quite possible for the palæontologist, by an appeal to the fossils alone, to arrange the series of sedimentary deposits into a pile of strata having a certain definite order. Not only would this