96 NEW YORK STATE MUSEUM 



varieties, corresponding to certain igneous rocks. There are thus 

 granite, syenite, diorite and gabbro gneisses, together with plenti- 

 ful intermediate varieties. Sometimes one variety will prevail 

 alone over a considerable area, but more commonly in Franklin 

 county two or more will occur in alternate bands, sometimes of 

 very slight width, rendering the areal mapping of the different 

 varieties an impossible task. 



A very significant fact in its bearing on the origin of these 

 gneisses is that in every case their mineralogy agrees precisely 

 with that of certain igneous rocks, and the same holds true of their 

 chemical composition in the few cases in which they have been 

 chemically examined. While this is by no means decisive evidence 

 in rocks so excessively metamorphosed, it seems improbable 

 that beds of the extent and thickness of these should have been 

 formed by deposition in water, with so> little change in composi- 

 tion, in other words from material so largely unweathered. These 

 rocks are thus sharply marked off from the characteristic Gren- 

 ville rocks, concerning which neither of the above statements is- 

 true. 1 



These gneisses are characterized in the large way by a rather 

 evenly granular structure and are usually fine grained, though 

 with considerable variation in this respect. Some of them show 

 traces of cataclastic structure, but this is the exception. Many 

 of them perfectly resemble the more finely grained portions of 

 rocks to be described later, which are regarded, both by the Cana- 

 dian and New York geologists, as of undoubted igneous origin. 



Orthoclase-quartz (granite) gneisses. The most widespread of 

 the gneisses is a fine grained, red, acid rock composed essentially 

 of alkali feldspars and quartz, with magnetite always present, 

 and usually small amounts of hornblende or biotite. The feldspar- 

 TS ordinarily microperthite or microcline, but some orthoclase is 

 always present and it may predominate, while a little acid 

 plagioclase is usually to be found in addition. Hornblende is the 

 usual dark silicate, biotite being much less common. 



lCf., Adams, F. D. Geol. sur. Canada. New series, v. 8, pt J. p. 35-85^ 



