I9I0.] PETRIFACTIONS AND ALTERATIONS. 23 



sthene has parallel extinction and characteristic pleochroism from 

 pale reddish to greenish white while the hornblende has has an 

 extinction angle* of about 14° and faint pleochroism from bluish 

 green to yellowish green. The mineral occurs in large anhedra 

 associated with light gray plagioclase. 



29. Sericite from Feldspars. — New York City. Specimens of 

 orthoclase and oligoclase from the pegmatites and pegmatite lenses 

 of the schists in the upper part of New York City are often ac- 

 companied by secondary muscovite or sericite in the form of thin 

 silvery scales. The scales occupy the cleavage planes but more 

 especially planes of parting parallel to the unit prism faces {no). 



* Determined on cleavage fragments. 



