THE PETROGRAPHY AND GENESIS OF THE 



SEDIMENTS OF THE UPPER CRETACEOUS 



OF MARYLAND 



BY 



MARCUS I. GOLDMAN 



Introductory 



The object of this chapter is to present the results of the detailed study 

 and the mechanical and microscopical analysis of a few typical sediments 

 from the Upper Cretaceous of Maryland. Work of this kind is merely 

 an extension of petrography to the sedimentary rocks; yet it has hitherto 

 been so little practised that most geologists hearing the term petrography 

 think instinctively of crystalline rocks. This comment is made in order 

 to forestall an attitude of mind towards what follows that is very gen- 

 eral, namely the belief that after such an analysis of a sedimentary rock 

 it is possible to determine the conditions under which the rock originated. 

 That is, of course, the ultimate object of such work, yet it is no more 

 implicitly the immediate result of the study of a given rock than the 

 study of a given crystalline rock in the beginning of that science was the 

 direct key to the origin of the rock — or is to-day, for that matter. If 

 decades of study of conglomerates, whose composition is apparent to the 

 unaided eye, leave many fundamental problems concerning this rela- 

 tively simple type of rock still unsolved, it is not to be expected that 

 microscopic knowledge of facts of the same kind about the sedimentary 

 locks of finer grain will suddenly reveal the conditions of their origin. 

 In fact, for these finer-grained rocks, as for the conglomerates, field study 

 of their larger geological characters, their variations vertically and hori- 

 zontally, the form of the whole mass, its relations to adjacent beds, and 

 other features must remain as important as the laboratory analysis. But 

 a more detailed knowledge of the composition of the finer-grained sedi- 



