352 FOSSILS 



Then it appeared that the supposed catastrophes, if they occurred 

 at all, were not general, but local, that records missing in Eu- 

 rope had been preserved, partially at least, in other continents. 

 Enough of these missing records has been recovered to show that 

 the earth's progress was not by a series of abrupt and sudden 

 changes, but by a continuous, orderly development. 



Major divisions of geological time are founded upon the more 

 striking changes in the animals and plants, while for minor divi- 

 sions the more detailed differences in the organisms are employed. 

 Parallel with the divisions of time run the groups or systems of 

 the strata, for characterizing which both the physical nature and 

 structure of the rocks and the fossils are employed. In the very 

 difficult and complicated task of compiling the earth's history, no 

 kind of evidence can be ignored, and wide knowledge and sound 

 judgment are needed in the work, so that no particular class of 

 records shall be either over- or under-valued. 



Though the goal of geological inquiries is to construct the his- 

 tory of the earth as a unit, this goal can only be reached by the 

 minute and exhaustive study of the local histories. Each of the 

 latter has certain peculiarities of its own which must be deter- 

 mined, and hence arises the multiplicity of local names for groups 

 of strata, so confusing to the student. Local names are useful, 

 because they avoid the necessity of premature comparisons and 

 correlations, which may lead to the direst mistakes. 



(2) As Evidence of Geographical Changes. — We have seen 

 that from the composition and structure of the stratified rocks 

 themselves much may be learned concerning the geographical 

 conditions under which they were formed, and of the subsequent 

 geographical changes of the region in which they occur. Fossils 

 supplement this information regarding the body of water in which 

 the rocks were laid down, whether fresh or salt, deep or shallow, 

 near or far from land, in an open sea or a closed basin, and whether 

 such a closed basin had occasional or constant communication 

 with the ocean. The stratified rocks which now form part of the 

 land surface give us information only concerning the former ex- 

 tension of bodies of water over what is now the land, but they can 



