180 The Petrography and Genesis of Sediments 



decompose this clay, as the initiation of the process, appears paradoxical 

 since the acid would first of all dissolve the shells forming the mould and 

 thus allow the as yet unaltered clay at once to disintegrate. 



These processes, moreover, seem to account for much of the glauconite 

 stain, that is, the glauconite forming patches and fissure fillings on and in 

 the grains of quartz and feldspar associated with glauconite. It is, of 

 course, possible that glauconite is formed as a fine powder from the loose 

 clay outside of any enclosing body, and it may well be this glauconite 

 that forms adhering patches on the outside of some grains. But any 

 fissure into which this could penetrate would surely be filled long before 

 by fine argillaceous material, so that here again it seems that the glau- 

 conite in the fissures of quartz and feldspar must be formed by the alter- 

 ation of an argillaceous product. The unusual thickness of some of these 

 seams in grains of feldspar, moreover, suggests that they are more prob- 

 ably derived from the alteration of kaolin formed in the fissure by the 

 alteration of the feldspar than from clay introduced from outside, since it 

 is very improbable that an open cleavage crack of that width would exist in 

 a grain of feldspar. 



Concerning the two closely related problems of inclusions in glau- 

 conite, and decomposition of the glauconite, the present observations 

 afford only confirmation of recognized facts. Thus the decomposition of 

 glauconite to yield limonite is generally accepted and is conspicuously 

 evident in the open-textured Monmouth sands. The clouded appearance 

 of the grains of these samples under the microscope is doubtless the result 

 of this process. The occurrence of clear, fresh-looking grains in the 

 samples of Matawan from the Chesapeake and Delaware Canal (samples 

 3 and 4) is on the other hand probably due to the protective action of the 

 clay in which they occur. 



Glauconite with inclusions of black grains (pyrite or magnetite 1 ) 

 were observed only in samples 8 and 11. In sample 8 it is note- 

 worthy that the micas, too, are full of black grains. ISTow, magnetite 



1 The differentiation of pyrite and magnetite from each other when they are 

 thus included in glauconite is, of course, difficult or impossible without chem- 

 ical means. 



