36 INTRODUCTION. 



Lastly, there are cases in which the distribution of fossil 

 organisms in different formations differs so much, or presents 

 such peculiarities, that we may reasonably suppose it to have 

 been conditioned by the special circumstances, perhaps now 

 undiscoverable, affecting the deposition of the strata of these 

 formations. Thus, in some cases, as, for example, in the 

 Carboniferous Limestone series, we find that the same fossils 

 characterise the entire series from top to bottom speaking 

 roughly, at any rate and that special kinds of fossils are 

 not restricted to special horizons in the series. This ap- 

 parent diffusion of the same kinds of fossils from the base 

 to the summit of a series of beds perhaps two or three 

 thousand feet in thickness, may of course be simply due to 

 the fact that we have not sufficiently investigated the organic 

 remains met with in the formation, and have not determined 

 with sufficient precision the exact horizons at which each 

 occurs. This is a work of time, and demands both great 

 stratigraphical knowledge and also a wide and accurate 

 acquaintance with the characters of the fossils themselves 

 two requirements rarely fulfilled in the same individual. 

 Still there are reasons for believing that in certain forma- 

 tions, the common and characteristic fossils range from the 

 top to the bottom of the series, so that it would not be pos- 

 sible to determine by means of the fossils the precise position 

 in the series of any given bed. On the other hand, there 

 are cases in which the fossils of a given formation may be 

 divided into two principal groups. In the one group is com- 

 prised a series of common forms of life w^hich may be re- 

 garded as characterising the formation as a whole. In the 

 other group are included certain special fossils which are 

 confined to particular parts of the formation, and which are 

 characteristic of certain definite horizons or zones within the 

 limits of the formation. All the great formations are to 

 some extent capable of being broken up into minor rock- 

 groups, characterised by special life-forms. Some of the 

 differences in the kinds of fossils found in different parts of 

 the same formation must, of course, be simply set down to 

 the fact that different kinds of sediment imply changed con- 

 ditions in the sea, and hence changes in the marine fauna. 



