H 



SEDIMENTATION, ENVIRONMENT, AND EVOLUTION. 5 



SEDIMENTATION, ENVIRONMENT, AND EVOLUTION 

 IN PAST AGES. 



When we review the two chief classes of rocks which 

 constitute the crust of the earth, the igneous or fire-formed 

 rocks, and the clastic or- sedimentary rocks, we cannot fail to 

 note the difference in the extent to which we know them. 

 The characters of the igneous rocks have been studied by 

 geologists in greater detail than those of the sedimentary rocks, 

 so much so, in fact, that the petrology of the former— their 

 origin, history, and the control of their mineral constitution 

 by physical laws — has reached the stage of philosophic treat- 

 ment. In contrast to this, the very wealth of organic remains 

 found in most of the clastic or sedimentary rocks has in many 

 cases diverted attention from their mineral and mechanical 

 characters and has left the field of the petrology of sediments 

 relatively obscure. On the one hand we find that the branch 

 of geological science which deals with igneous rocks is becoming 

 more and more amenable to mathematical treatment ; on the 

 other hand, the study of sediments has only in recent years 

 become quantitative. 



It is one of the tenets of geology that the present is the 

 key to the past. But in many cases we have no certain informa- 

 tion as to the conditions under which ancient sediments were 

 laid down ; criteria are lacking because our acquaintance with 

 the mode of formation of similar deposits at the present day is 

 insufficient and inexact. This lack of knowledge is the more 

 to be regretted since, apart fron the features displayed by the 

 entombed -fossils themselves, the nature and character of the 

 sediments provide the only clues to the environments in which 

 the organisms dwelt, and which may have caused or modified 

 the evolution of lineages. The environmental conditions would 

 not always leave their impress upon the rocks, but it is probable 

 that many which did so have left traces that are insufficiently 



