112 The Petrography and Genesis of Sediments 



mentary rocks is desirable than the current terms, sandstone, shale, sandy 

 shale, tuff, limestone, or even more circumscribed terms like chalk, green- 

 sand, etc., afford; and from the awakening interest in this subject it is 

 safe to expect that before long every stratigraphic study of a limited 

 region will contain descriptions of the composition of the sedimentary 

 rocks involved. Every such study will bring out some significant facts 

 regarding the origin of the particular rocks, but for a satisfactory final 

 interpretation of the conditions under which the rock originated it will be 

 necessary to have accumulated an extensive series of analyses of modern 

 sediments of all possible varieties. Comparing then the ancient sediment 

 with the modern ones, the conditions of whose origin will be more or less 

 completely known, it will be possible by finding the modern sediment that 

 is most similar to determine the conditions under which the ancient sedi- 

 ment in question was formed. On the other hand, the sediments of the 

 past offer some opportunities to the investigator that are lacking in the 

 modern. For in the subaqueous sediments of to-day only what is at the 

 surface, or a few feet below, can be examined. Of the ancient sediments, 

 however, it is possible to obtain sections in which the changes both vertical 

 and lateral can be followed out, and thus knowledge gained which could 

 be gathered from sediments in process of formation only through cen- 

 turies of observation or through periods too long for consideration. Thus 

 the two branches of the study must advance together, each throwing light 

 on the facts of the other, and the two pointing out to each other the 

 problems that require special attention. 



It is this consideration that has led to the attempt to interpret freely 

 the facts obtained in the present study in the belief that an investigation 

 is valueless until some conclusion has been drawn from it, and that the 

 investigator who has accumulated the facts is in the most advantageous 

 position for interpreting them. These interpretations, however, are put 

 forward most tentatively and with the greatest possible reservation. 



While the published literature describing modern sediments is not 

 inconsiderable, 1 it is not of much value for the Cretaceous sediments 



1 For a very full and up-to-date bibliography, see Andree, K., Ueber Sediment- 

 bildung am Meeresboden Geol. Rundschau, vol. 3, 1912, H 5/6, pp. 324-338. 



