GRANTIT.AR QUARTZ. 



159 



thermal wafers still exist, though it docs not appear that they deposit any thing but tufa at 

 present ; 3d, as has been stated above, it is homogeneous and similar throughout, though 

 rather coarser than the siliceous rocks which we know have been deposited from thermal 

 waters ; 4th, it occurs apparently in insulated beds, which bear the semblance of having been 

 made by some local cause. The gi'anular quartz is traversed by veins of white amorphous 

 quartz, which appear to have some degree of regularity in the direction which they pursue. 



The color of the rock is usually brown ; upon the outside lighter, or grey, upon a recent 

 fracture. Beds or masses are sometimes snow-white and friable, passing by disintegration 

 into a sand with a sharp grit. 



The varieties which I have observed, are, 1st, the common hard rock, crystallized into 

 rhombic prisms ; 2d, the distinct white granular kind, which ultimately becomes sand ; 3d, 

 a compound rock in which feldspar exists in small crystals of white or yellowish-white color. 

 The latter is subject to decomposition ; the feldspar disappears, and leaves a porous quartz, 

 which has been employed for millstones, and which slightly resembles buhrstone. This variety 

 has been largely formed in some localities, as in Pownal (Vt.), where a large bed of porcelain 

 clay has been found, which probably owes its origin to the decomposed rock. 



The relation which this rock holds to the others belonging to this system is exhibited in the 

 following section, which extends from Adams to the base of the Taconic ridge, over Oak hill, 

 in Williamstown : 



sit 



1. Hoosic mountain, composed of gneiss. 



2. Stockbridge limestone, valley at Adams. 



3. Oak hill, of granular quartz. 



4. Limestone. 



5. Magnesian slate. 



The dip and strike of the beds of this rock conform to the other rocks of the system ; the 

 the strike being N. 10° W., and the dip 30° E. 



On Stone hill, the great mass of quartz is divided by a thick bed of dark blue siliceous 

 slate. This slate is tolerably even-bedded, and answers a good purpose as a flagging stone. 

 This hill is about one and a half miles in length, and arises out of the Hoosic valley in a 

 perfectly insulated state. It is about four hundred feet high, and presents upon the west 

 side a high mural front of hard granular quartz. In the line of direction, another mass of 

 this rock appears, but it rises only just above the surface, and is quite inconspicuous. 



A more important bed or mountain range lies north of the road leading from Williamstown 

 to Adams. This mountain is fifteen hundred feet high, and apparently extends fifteen or 

 twenty miles north, preserving its mountainous character, and rising apparently still higher in 

 a ridge between Bennington and Woodford (Vt.). 



