98 NEW YORK STATE MUSEUM 



Pyroxene gneisses, or granulites. These rocks are usually found 

 interbanded with the granitic gneisses in Franklin county, the 

 bands ranging from a few inches to many feet in thickness. 

 Occasional large masses occur but are rather exceptional. They 

 are gray to black, rarely red, in color and consist of pyroxene 

 (both augite and hypersthene usually) and plagioclase and ortho- 

 clase feldspars as essential constituents, often with hornblende 

 as well. Quartz is present at times. Petrographic descriptions 

 of several of the varieties which these rocks present have been 

 recently published by Adams, and need not be repeated in a report 

 of this preliminary character. 1 The rocks are rather evenly 

 granular, show little or no sign of cataclastic structure, and vary 

 considerably in mineralogy from place to place. 



These granulites are mainly confined to the northern part of 

 the county, where they occur interbanded with orthoclase 

 gneisses, or occasionally in rather large masses. In Malone and 

 Belmont they are of gray color and consist of pyroxene (both 

 hypersthene and aegirin-augite), orthoclase and a plagioclase 

 which varies from oligoclase to andesine. If present at all, horn- 

 blende is in only slight amount. Sometimes the orthoclase, at 

 others the plagioclase feldspar predominates, and each may 

 appear to the exclusion of the other. Similar rocks are wide- 

 spread throughout the Adirondack region. 



In Brandon and Dickinson better foliated, black gneisses, com- 

 posed essentially of hornblende and andesine feldspar, accompany 

 the orthoclase gneisses, and the pyroxene granulites are scarce. 

 These hornblende gneisses frequently contain augite or hypers- 

 thene or both, thus showing a gradation into the pyroxene rocks. 



These gneisses are also extremely common in the Adirondacks. 

 They have the mineralogy of diorites, or gabbro-diorites. They 

 are absolutely not to be distinguished from hornblende gneisses 

 of frequent occurrence which are unquestionable derivatives of 

 the gabbros, described on a later page. In many localities such 

 rocks are found containing an unchanged core of the igneous rock 

 with characteristic ophitic structure, from which a gradual trans- 



1 Geol. sur. Canada. An. rep't, v. 8. pt J. p. 73-82. 



