22 Economieal Geology. 



The hornblende slate* in various parts of the State, but particular- 

 ly in the region of the Connecticut River, is frequently porphyritic ; 

 and exceedingly resembles porphyritic greenstone ; being, in fact, 

 composed of the same ingredients ; and differing only in its slaty 

 structure, and in the more distinctly crystalline character of the horn- 

 blende. The disseminated crystals of feldspar are usually white. In 

 Canton and Easton, they are sometimes the compact variety, yet re- 

 taining their form perfectly. A fine variety and in large quantity oc- 

 curs in Heath, a specimen of which may be seen in the collection. 



The magnetic iron ore in Cumberland, {R. I.) is profusely sprink- 

 led with crystals of feldspar ; and would doubtless form no mean sub- 

 stitute for green or black porphyry. 



, 



Quartz Rock. 



When this rock occurs pure, it can hardly be employed in archi- 

 tecture of any kind, on account of its breaking into fragments so ex- 

 tremely irregular. But when it takes a small proportion of mica into 

 its composition ; it is often divided, with mathematical precision, into 

 layers of convenient thickness for building. The best quarry of this 

 kind that I know of, is in the west part of Washington, Berkshire 

 County, about three miles south-east of Pittsfield village. The lay- 

 ers vary in thickness from one or two inches, to one or two feet; 

 thus affording materials for fine flagging stone, as also for walls and 

 underpinning. The quantity of this rock at the quarry is very great. 



Although quartz rock is usually, of all others, most easily affected 

 ,by heat, yet that variety from the quarry in Washington, is remarka- 

 ble for its power of resisting heat ; and it is here .employed for the 

 hearths and walls of furnaces. Prof. Dewey says that he has " seen 

 this stone after it has sustained the highest heat ,of .the furnace for 

 months, and found its surface merely glazed by the high tempera- 

 ture." It was transported to the iron works in Bennington, Vt. until 

 a similar rock was discovered in that town. It occurs also in Wil- 

 liamstown. What peculiarity this rock possesses, that renders it able 

 to resist a high temperature, I do not know. 



Another valuable variety of quartz rock is found near the quarry 

 .above mentioned. But its use, as well as that of another variety in 

 Cheshire, will be noticed subsequently. 



