PALEONTOLOGY. 157" 



L I IJ K A U 1 



UNIVERSITY < 



CALIFORNIA 



CHAPTER 



Characteristic Fossils Table of Fossils. 



Characteristic Fossils. The interruptions of continuity 

 in the conditions under which the rocks were formed,, 

 and the resulting variations in their enclosed life-remains,, 

 afford a means for their classification. They are thus 

 divided into great groups or systems, representing life- 

 periods and eras, corresponding to such divisions and 

 sub-divisions. 



"Each of the great groups has an assemblage of 

 fossils peculiar to it, so that the general assemblage 

 of fossils found in one group is not found in any other 

 group, either above or below. As minor exceptions to 

 this rule, particular species of fossils seem occasionally 

 to range in to two, or perhaps even three, adjacent groups, 

 occurring perhaps in the upper part of one group, rang- 

 ing through the whole of the group above it, and appear- 

 ing in the lower part of the group above that. Some 

 species, on the other hand, are found only in a very small 

 part of one group, either throughout the lateral extension, 

 of the beds wherever they occur, or sometimes limited 

 to some small locality in those beds. 



" When a single species or an assemblage of several 

 species occurs in a group of rocks, whether large or 

 small, and has never been found except in that group- 



