PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 35 



Some fine examples of crudely lens-shaped masses of practically 

 pure silica (silexite) have been observed by the writer in the Blue 

 Mountain quadrangle in syenite, granite, and mixed gneisses 

 apparently always parallel to the foliation of the inclosing rock. 

 In one case a small dike of pegmatite sharply cuts a long, narrow 

 lens of silexite. 



Earlier pegmatites. — A great many masses of true pegmatite 

 of early origin lie in the Lyon Mountain granite parallel to its 



■ ■:■:■■.'■ ^3 1 kzite l^^^O^anife 



Diabase 



root 



Fig. 5. — Angular inclusions of both silexite and Grenville quartzite in foliated 

 granite at the top of the hill one-third of a mile south-southwest of Barnes Pond in 

 the Lyon Mountain quadrangle. 



foliation. The mineral content of these early pegmatites is remark- 

 ably simple, by far the greater number of them consisting very 

 largely of quartz and potash feldspar (mostly microcline), some 

 microperthite and oligoclase, and a Httle biotite. Some pegmatites, 

 probably to be classed with the early ones, though not with the 

 earhest, carry more or less magnetite, hornblende, or pyroxene, 

 but these minerals appear to have been largely or wholly derived 

 from the old dark gneisses and gabbro which were invaded by 

 the granite. Minerals so common to pegmatites in general, such 

 as tourmaline, garnet, muscovite, beryl, apatite, and others, must 

 be exceedingly rare if they occur at all. The only mineralizer, 



