168 The Petrography and Genesis of Sediments 



which fossils are particularly abundant, indicating fluctuations in the 

 conditions of deposition. 



In summing up it becomes necessary to neglect the diagram entirely 

 and to rely on the general physical characters. Here the evidence of much 

 humus, the high sulphide content, and the peculiarities of the fauna 

 are indications of a very stagnant, brackish body of water, perhaps sur- 

 rounded by swamps or filled with disseminated algal growths. The 

 region was evidently near enough to some stream to be affected by fluctu- 

 ations in its transporting power resulting in the separation of sand layers 

 by clay films and layers of fossils. But the best explanation for the 

 peculiarities of the diagram of this sediment is in just these secondary 

 grains. It is their development that can account for the coarseness of the 

 sand, and one may even assume that they had reached a certain average 

 size, between 1 mm. and -i mm., to account for the maximum in that size. 

 This interpretation is very hypothetical, but it is the best combination 

 that presents itself for the various partly conflicting factors that are 

 involved. The questions presented require more detailed and extensive 

 study. 



General Summary and Conclusions 



The special features of each sediment having been discussed, it remains 

 to sum up the conclusions arrived at and to give a general review of the 

 glauconite in the different samples. 



classification of the sediments 



In the discussion of the 13 sediments studied in this paper three types 

 have been differentiated: (1) The delta type; (2) the estuarine or 

 lagoonal type; (3) the open- water glauconitic type. The character of 

 each may be briefly summarized as follows : 



The delta type has as its foremost characteristic the large proportion 

 of a wide range of sizes of sand which a single sample taken from it con- 

 tains. In the diagram this is expressed by a broad curve with no pro- 

 nounced maximum. This character is not very markedly affected by the 



