50 



GEEEN MOUNTAINS IX MASSACHUSETTS. 



grained, closely interlocking aggregate of little rounded or irregular quartz 

 grains mixed with considerable feldspar in similar irregular grains. Broken 

 or rounded crystals of apatite and zircon and perfect crystals of tourmaline 

 and rutile are common. The feldspar grains are in part microcline, 

 plagioclase, and an untwinned feldspar (orthoclase?). Unless it he the 

 apatite and zircon, no unmodified original clastic elements can be recognized 

 in this rock. According to the usual view of the origin of quartzite, the 

 quartz grains have been enlarged by the growth of new silica, so that the 

 original form is wanting, and the feldspar, judging from its similarity to 

 that of other rocks in which it is undoubtedly metamorphic, has probably a 

 similar origin. 



In many localities the quartzites have a crumbly character, so that 



Fig. 15 — Metamorphic conglomerate (Vermont formation), near contact with granitoid gneiss. Top of Hoosac moun- 

 tain. Fallen block. One-twentieth natural size. 



This also shows the production of flattened " pebbles " by crushing and tin- development of biotite, etc., along crushing 

 and slipping planes. In the right hand »»1 the picture this is especially clear. The pebbles here are granulite, passing 

 into a fine grained granite. 



they can be picked or shoveled out, and are extensively quarried for glass 

 sand. Prof. J. I). Dana has called attention to this 1 and suggested weather- 

 ing as a cause, and connected it with the alteration and leaching out of the 

 feldspar. In some of the quarries the percolating water carries down tine 

 kaolin, and forms beds of pipe clay in the bottom of the quarry. But some 



1 tin the decay of quartzite, and the formation of sand, kaolin, and crystallized quartz. Am, 

 .lour. Si i.. 3d set., vol. 28, 1884, p. 448. 



