GEOLOGY OF THE LAKE PLACID REGION 55 



bed of the stream. A third is in a hill a mile east of Scotts Cob- 

 ble, and the fourth is in the bed of the East Branch at the cas- 

 cade between Keene Center and the iron bridge a mile and a half 

 south. In each case the rock is chiefly quartz and feldspar, in a 

 coarsely crystalline aggregate. In thin section the quartz is 

 found to be much strained from geologic movements, and the 

 feldspar is the variety microperthite, being an orthoclase thickly 

 set with little spindles of albite. 



The three rocks above referred to, and the trap' dikes to be 

 later described are minor rock formations, nearly all the country 

 being made up of the two that follow next. 



Gneiss. Under this comprehensive name is included a consid- 

 erable variety of rocks, all of which have however the distin- 

 guishing feature of ' gneiss ' in strong development. That is, the 

 light and dark minerals are arranged in rudely parallel bands so 

 as to give a foliated or laminated aspect to the rock. The 

 banding varies from coarse to fine, and is produced by innumer- 

 able flattened lenses of minerals, strung out with their long 

 diameters parallel. The bands curve and eddy at times and 

 strongly simulate the phenomenia produced by the flow of a ropy 

 or viscous fluid. 



The commonest gneiss is a dark, and more or less rusty rock, 

 with abundance of black minerals, set in a brown or green 

 mass of feldspar. When a fresh exposure is produced either by 

 pounding to the core of a large fragment or in blasting boulders 

 and ledges for the improvement of highways, it is seen that the 

 rock is a pronounced green. Eed garnets are frequently quite 

 prominent in it. In thin sections this variety is found to be 

 chiefly co'mposed of microperthitic feldspar and emerald green 

 pyroxene. With these in places here and there are varjang 

 amounts of hornblende, hypersthene, quartz, garnet, magnetite 

 apatite and zircon. W^hen the quartz is abundant, varieties high 

 in silica result; when it and the feldspar are in less amount, dark 

 pyroxenic and hornblendic varieties occur in consequence. 



In some gneisses large blue labradorites are quite prominent, 

 but always in rudely lenticular form, giving the general impres- 



