166 The Petrography and Genesis of Sediments 



are found still to contain the black flakes which proves that the black material is really 

 on the inside. Grains of the same kind were picked out of the medium-grained sands 

 (the dark-gray grains mentioned in the hand-lens description). Some of these were 

 composed of colorless quartz, others showed a humus-brown color throughout. They 

 also polarized as units and on crushing showed the same dissemination of the black 

 flakes throughout the original grain. I have, therefore, concluded that these grains are 

 secondary, that is, formed after the deposition of the bed. 



(2) Heavy 



Almost half of this portion appeared to be magnetite, and red garnet is very common. 

 Rutile is also common. 



Rarer. — Epidote, tourmaline, pyroxene, chlorite, enstatite, zircon, sillimanite (?). 

 Some of the garnet and epidote are well rounded. 



11. Extra Fine 



A very dark, brownish-gray, fine-grained, very slightly micaceous powder. This material 

 is finer than in most of the samples because it contains much that usually goes into the 

 silt. Under the microscope it shows much argillaceous matter in brown floccules. 



Many small, irregular roundish to perfectly spherical nodules of marcasite. Some of 

 the black nodules of marcasite are fringed by a brown, translucent, isotropic substance. 

 In other cases they are made up of an agglomeration of tiny spherules in a matrix of 

 such substance. There are some chloritic, perhaps a few glauconitic fragments ; in 

 addition of course many quartz and feldspar grains. 



III. Clay 

 Dirty brownish-gray. It contains much of the dirty, fibrous, polarizing material besides 

 the usual amorphous brown flocculent matter, and some mineral grains. 



Summary and Conclusions. — This is a very peculiar and distinct sedi- 

 ment and must be the product of special conditions which are only partly 

 brought out by the above study, so that no attempt will be made to do 

 more than indicate some of the factors in its origin. The peculiar impres- 

 sion it makes is probably due mainly to its coarseness, its truly black 

 color, its very friable condition, due perhaps to the fact that the black 

 " clay " binder (it is not abundant enough to form a matrix) is not true 

 clay, i. e., not colloidal, or else that the peculiar conditions under which 

 it was deposited destroyed its coherence. The abundance of sulphide 

 (presumably marcasite) and coarse brackish-water features of the fauna 

 sustain the impression of something unusual. One would say a very stag- 

 nant lagoon, estuary or delta, yet the diagram (M, p. 169) does not bear 

 this out, for it suggests good sorting, quite as good, excepting for the clay, 

 as in the open-water marly Monmouth (sample 11, K, p. 169) . But in con- 

 sidering the sizes involved it is noticed that there is in all the diagrams 

 presented not another one (even marine beach sand) which has the maxi- 

 mum in a portion so coarse as the medium sand (li mm.) It might be 



