300 NEW YORK STATE MUSEUM 



have the composition of granites. They vary much in coarse- 

 ness from place to place and from band to band, ranging from 

 finely and evenly granular varieties (which are the prevailing 

 ones) to those which are quite coarsely crystalline. An alkali 

 feldspar (microperthite, microcline or orthoclase in order of 

 abundance) and quartz are the prevailing minerals, magnetite 

 is always present, and usually hornblende in small amount as 

 well. Biotite sometimes occurs instead of hornblende or together 

 with it. The coarser gneisses often show traces of cataclastic 

 structure, larger individuals, usually of feldspar, being sur- 

 rounded by a granular mosaic which seems to have resulted, at 

 least in part, from the crushing of the larger individuals. The 

 fine grained varieties have the character of this mosaic through- 

 out, the larger individuals being absent, so that, though they are 

 likely akin to the coarser rocks, being simply more thoroughly 

 granulated representatives, due perhaps to original finer grain, 

 it is impossible to be certain that this is the case. 



Along with these red gneisses, often interbanded with them 

 with seeming regularity, often found in large masses, are two 

 other kinds of gneiss of common occurrence. Like the red gneiss, 

 they show many variations in composition and appearance, and 

 also show a rude foliation, usually parallel to the banding. The 

 more abundant sort is usually gray in color and consists essenti- 

 ally of pyroxene and feldspar, both plagioclase and orthoclase. 

 The pyroxene may be either augite or hypersthene (or enstatite), 

 or both, augite being the more frequent. The usual augite is a 

 deep emerald green and shows pleochroism from green to yellow 

 green, resembling aegirine augite. Magnetite is always present 

 and sometimes a little hornblende and biotite as well. Either 

 orthoclase or plagioclase may be in excess and either may be 

 present to the exclusion of the other. The plagioclase is usually 

 oligoclase but sometimes andesin. These rocks usually show an 

 evenly granulated or granulitic structure quite like that of the 

 fine grained red gneisses, but how much this is due to granula- 

 tion, or to recrystallization, or may even be original, can not be 

 determined. 



