OR GRANULAR QUARTZ. 85 



1 can not but regard the Berkshire quartz rock as a member of the Taconic system. It is 

 constantly associated with those rocks which I consider to be members of this system, and 

 it corresponds rather in dip and strike with the limestones and slates of the same, while it is 

 certainly unconformable to the gneiss and mica slate. Thus, at Arlington, Vermont, the 

 quartz dips N. 20° E. at an angle of 10° - 15°, resting upon the edges of the highly inclined 

 gneiss. Facts of this kind have influenced me to associate the quartz with the Taconic 

 system. How it happens that this rock is in juxtaposition with gneiss, or apparently 

 passes into it, I shall try to explain in a subsequent section. 



I have not been able as yet to form an estimate of the thickness of the quartz. The 

 largest mass, and which reposes against and upon the gneiss of the Hoosic range, can not 

 be less than one thousand feet thick. The more westerly mass, and which forms Stone 

 hill in Williamstown, is probably about three hundred feet thick. I give these, however, 

 as only rough approximations. 



MINERAL CONTENTS. 



Scarcely any rock is so destitute of mineral bodies as this. The whole catalogue seems 

 to be confined, so far as my observations go, to a little sulphuret of iron, sometimes in 

 simple unmodified cubes ; in other instances, disseminated in fine particles through the 

 mass. Quartz veins, whose characters are different from the main rock, often traverse it 

 in parallel seams. In the slaty variety, however, where it is near the line of contact with 

 gneiss, I have observed needleform schorl. We have, therefore, in this quartz rock, 

 another instance of one that is quite barren and uninteresting so far as mineral products 

 are concerned ; and it agrees in this respect with most of the pure siliceous deposits, for 

 they of all others appear to be the most destitute of veins and valuable metalliferous pro- 

 ductions. 



RANGE AND EXTENT. 



There is more difficulty in tracing the range of the Granular quartz, than that of any 

 of the other members of the Taconic system. I have already remarked that it often ap- 

 pears in heavy mountain masses ; but these are quite limited in extent, and disappear in 

 the direction of the strike, suddenly and unexpectedly for such heavy and important strata. 

 This fact has suggested the inquiry whether it exists really as an independent rock ; and 

 on this point, Prof. Dewey remarks,* that it may occur only in beds in mica slate; but 

 as it occurs in great quantities, he treats it as a principal rock. He speaks of it, too, as 

 occurring on both sides of the Stockbridge limestone. The hills of quartz run parallel 

 with the general range. The mountain northeast of Williams College, Massachusetts, is 

 fourteen hundred feet high ; and being followed eastwardly over its summit towards 



' History of Berkshire, p. l'Jt, Article Geology. 



