Maryland Geological Survey 181 



is a decomposition product of biotite, and biotite may also be bleached or 

 converted into chlorite, so that the micas present in this sample might 

 all be derived from the decomposition of biotite. On the other hand, 

 this bed is also sulphide-bearing. Cayeux has suggested that pyrite and 

 magnetite might be introduced into glauconite grains subsequent to their 

 formation, but not, presumably, in a loose sediment of this kind. Collet 

 notes (p. 160) that those inclusions in glauconite are more common in 

 ancient than in modern sediments. Might not these black grains, then, 

 be magnetite produced by decomposition of the glauconite as it is pro- 

 duced in biotite? 



There is one fact specially noteworthy about the glauconite sands of the 

 Monmouth, that is, the coarseness of the accompanying sand. The asso- 

 ciations in the Matawan are normal since Thoulet found it even in the 

 narrow coastal strip of the Gulf of Lyon which he studied, 1 but its occur- 

 rence in sediments as coarse as these (in fact as the whole Monmouth and 

 Eocene of this region) is not recognized in modern sediments. On the 

 other hand, there is no theoretic reason against such an association. 



According to Collet the feldspars associated with glauconite are pre- 

 dominantly basic, of about the composition of labradorite. While no spe- 

 cific identification of the feldspars present was made the writer's observa- 

 tions do not at all confirm this conclusion. The twinning characteristic of 

 plagioclase feldspars was exceedingly rare, and the index of refraction of 

 the feldspars was, moreover, almost invariably lower than that of the 

 liquid (1.548) in which they were immersed, which would imply nothing 

 more basic than oligoclase. These observations do agree, however, in that 

 orthoclase seemed to be scarce. 



The degree of weathering of the feldspars in the glauconitic samples is 

 very variable, and is in these ancient sediments doubtless determined 

 largely by secondary effects after their exposure. This belief is con- 

 firmed by the fact that feldspars are scarcest in those samples (9 and 

 11) which show clearly their derivation from the erosion of a deposit 

 previously formed, which in the interval before it was reworked must 



1 Thoulet, J., Etude bathylithologique des cotes du Golfe du Lion. Annales 

 de l'lnst. Oceanograph. T. iv, Fasc. 6, Paris, 1912, p. 62, et seq. 



