172 The Petrography and Genesis of Sediments 



aceous matter occur, and the high percentage of magnetite is sometimes 

 noticeable. 



Under the glauconitic-sand type only the three samples, 7 and 8 

 from the Matawan, and 12 from the Monmouth, will be considered. 

 Foremost among the characters of the glauconitic sands is their coarse- 

 ness and the accompanying low percentage of clay. The figures for the 

 clay unfortunately do not bring this out as clearly as they should, on 

 account of the great amount of ocherous matter present, which tends to 

 be separated with the clay. With these striking characters goes better 

 sorting of the sands, that is, a more sharply defined maximum in the 

 diagram, and generally a lower proportion of heavy minerals (the glau- 

 conite having been deducted in these samples on the assumption that it 

 was formed in place). 



What is called the estuarine type lies between these two other types, 

 and therefore, naturally, shows transitions to both of them. Thus, 

 sample 5, the sandy yellow glauconite bed in the Matawan, would, but 

 for its associations, be classed unhesitatingly with the group of glau- 

 conite sands. Indeed, an estuary or lagoon from its very nature can 

 readily become an open body of water, and there is no reason why this may 

 not be assumed to have happened here. There is the characteristic sorting 

 of the sands, the only difference from the other glauconitic sands being 

 the greater fineness of the maximum size ; but there is no reason for believ- 

 ing that such a character cannot belong to a typical glauconite sand; and 

 the limited number of analyses of typical glauconite sands does not justify 

 making a contrary generalization. 



The most conspicuous feature of what is called the estuarine or lagoonal 

 type is of course the characteristic black, argillaceous appearance of the 

 Matawan, by which it is so readily recognized in the field. The cause of 

 this coloring is one of the unsolved problems in the study of these deposits. 

 In the normal samples of this type the clay itself, when separated, is of the 

 ordinary blue-gray color. The black color cannot be attributed to organic 

 matter since that is, in the most characteristic samples, not unusually 

 abundant, and moreover, it may be seen from the Magothy that the pres- 



