﻿32 NEW YORK STATE MUSEUM 



of massive, coarsely crystalline quartz, running up to as high as 

 90$ of the rock, though feldspars and accessory minerals are always 

 present. The thinner quartzite beds are generally more impure, 

 though containing layers of coarse, massive, quite pure quartzite. 

 The impurer beds are often well foliated, consisting of alternate 

 films of pure quartz and of other minerals, the former very resistant 

 to the weather, the latter less so, so that on the weathered surfaces 

 the contortions and puckerings of the complexly folded schist 

 series are much more perfectly displayed than in any other rock 

 type of the region. They are often very close jointed, especially 

 near granite, weathering out into small blocks [pi. 3]. 



Much of the quartzite of the district is more or less permeated 

 with brown, iron-stained spots, due to the weathering out of some 

 mineral with iron in its composition. These spots vary greatly in 

 abundance in different occurrences and different layers, and may 

 have a fairly uniform distribution, or, in the foliated varieties, be 

 confined to the films containing other minerals than quartz, giving 

 a brown and white, banded rock. In some cases, notably those of 

 the first type, the mineral removed seems to have been pyrite, a 

 mineral of consistent occurrence in the quartzite; in other cases 

 it seems to have been pyroxene, though even here probably oxidized 

 pyrife was responsible for most of the yellow, iron stain. 



In texture the rock shows great variation, ranging from the very 

 coarsely crystalline, glassy rocks, down to varieties which have a 

 finely granular make-up. 



Next to quartz, feldspars form the most prominent mineral con- 

 stituent, orthoclase, microperthite and oligoclase all occurring. 

 Much variation in relative amounts of the two mineral groups is 

 shown, but in the great hulk of the rock, quartz is in excess and 

 usually greatly in excess. In some varieties white to light green 

 pyroxene appears in quantity, when the feldspar retreats. There is 

 considerable of such quartz-pyroxene gneiss in the region, the 

 quartz usually constituting 75^ of the rock. Light brown mica 

 (phlogopite) is sparingly present in much of the quartzite, and some 

 varieties become quite micaceous. Pyrite is a frequent mineral, 

 as has been stated. Zircon and titanite are nearly always present, 

 and at times fine needles of rutile are abundantly included. 



Here and there in the region rocks are found which present a 

 puzzling half way stage between quartzite and granite, so that they 

 are likely to be classed, now with one rock, and again with the 

 other, according as the observer comes upon them from quartzite, 

 or from granite. In all cases where the relations could be worked 



