38 PRINCIPLES OF PALAZZONTOLOGY. 
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 “at 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 paleontologist, 
by an appeal to the fossils alone, to arrange the series of sedi- 
mentary deposits into a pile of strata having a certain definite 
order. Not only would this be possible, but it would be found 
—if sufficient knowledge had been brought to bear on both 
sides—that the paleontological arrangement of the strata would 
coincide in its details with the stratigraphical or physical 
arrangement. 
Happily for science, there is no such division between the 
paleontologist and the physical geologist as here supposed ; 
but by the combined researches of the two, it has been found 
possible to divide the entire series of stratified deposits into a 
number of definite vock- groups or formations, which have a 
recognised order of succession, and each of which is charac- 
terised by possessing an assemblage of organic remains which 
do not occur in association in any other formation. Such an 
assemblage of fossils, characteristic of any given formation, re- 
presents the fe of the particular pevzod in which the formation 
was deposited. In this way the past history of the earth 
becomes divided into a series of successive /éfe-periods, each of 
which corresponds with the deposition of a particular forma- 
tion or group of strata. 
Whilst particular assemblages of organic forms characterise 
particular groups of rocks, it may be further said that, in a 
general way, each subdivision of each formation has its own 
peculiar fossils, by which it may be recognised by a skilled 
worker in Paleontology. Whenever, for instance, we meet 
with examples of the fossils which are known as Graffolites, we 
may be sure that we are dealing with Sc/wrian rocks (leaving 
out of sight one ortwo forms doubtfully referred to this family). 
We may, however, go much farther than this with perfect 
safety. If the Graptolites belong to certain genera, we may 
be quite certain that we are dealing with Zower Silurian rocks. 
Furthermore, if certain special forms are present, we may be 
even able to say to what exact subdivision of the Lower Silu- 
rian series they belong. 
As regards particular fossils, however, or even particular 
classes of fossils, conclusions of this nature require to be accom- 
panied by a tacit but well-understood reservation. So far as 
