CHRONOLOGICAL SUCCESSION. 37 
GHA PTE R4E8E 
CHRONOLOGICAL SUCCESSION OF THE 
FOSSITIFE ROO S: ‘ROCESS. 
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 cer- 
tain 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 pos- 
sible, therefore, for the physical geologist to arrange the known 
stratified rocks into a successive series of groups, or “ forma- 
tions,” having a certain definite order. The establishment of 
this physical order amongst the rocks introduces, however, at 
once the element of me, and the physical succession of the 
strata can be converted directly into a historical or chronologt- 
cal 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 de- 
termine in each country the general physical succession of the 
strata, and this “ stratigraphical” arrangement, when once de- 
termined, 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 paleontology 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 suc- 
cession of the strata in any given region, but it becomes now 
for the first time possible to compare, with certainty and pre- 
cision, 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 estab- 
lished law of Palzeontology is, that particular kinds of fossils 
