468 NEW YORK STATE MUSEUM 



this scale. There are various other gneisses present which are un- 

 like anything found in the Long lake gneiss, except in the near 

 vicinity of Grenville belts, most of which seem igneous, though 

 some are of doubtful origin. Some of these gneisses are easily recog- 

 nizable, others are discriminated from the Long lake gneiss only 

 with difficulty. Along with all these is a general matrix of Long 

 lake gneiss. 



There is, for example, much of a black and white gneiss, which 

 consists of hornblende or pyroxene and plagioclase feldspar, either 

 andesin or labradorite, with accessory apatite, magnetite, titanite 

 and zircon. In most of the occurrences the feldspar predominates, 

 and the rock is spotted in appearance and fine grained. With 

 increasing hornbleide the grain becomes coarser and the rock is 

 striped instead of spotted. Often one variety is interbanded with 

 the other. A similarly appearing rock in the field shows pyroxene 

 instead of hornblende, with much titanite and a more acid feldspar 

 (oligoclase) . These rocks have the mineralogy of gabbros and dio- 

 rites, but the field appearance is often suggestive of a sedimentar}^ 

 origin. There is often a strong resemblance to the rock of the 

 " Whiteface " region which Kemp has described as the " White- 

 face type " of anorthosite.^ That rock behaves at times like an 

 intrusive, at others strongly suggests a sediment, and its true nature 

 and relations have not been clearly made out. If an igneous rock, 

 its customary Grenville association has not been explained, and a 

 close association in age is indicated; an age older than that of the 

 ordinary anorthosite. 



Another gneiss in this group is a red, usually acid, rock composed 

 of quartz and alkali feldspar, with a considerable content of green 

 pyroxene and a deep colored titanite. This rock is also quite vari- 

 able and is a frequent rock in the Adirondack region, often associated 

 with magnetite deposits, as at Lyon Mountain. Its true nature, 

 association and age are yet to be discovered. 



There is also found much of a peculiar granitic rock, differing in 

 appearance from the ordinary Long lake granitic gneiss, the dif- 

 ference being difficult to describe, though easy to recognize. It is 

 a rock of medium grain, not extra gneissoid, much lighter red than 

 the Long lake gneiss, and contains from 10 per cent to 20 per cent 

 of hornblende, magnetite and biotite. It occurs in a gi-eat number 

 of Grenville sections, lying in among the sediments, or cutting them 



I N. Y. State Geol. isth An. Rep't. 1895. 1». 587. 



