HOOSAC MOUNTAIN. 



55 



the granitoid gneiss, and the mica is black. In the slides the structure is 

 essentially the same as that of the granitoid gneiss: the large crystals are 

 microcline, broken, faulted, tilled with fluid inclusions, epidote grains, 

 quartz, mien, etc.; while the groundmass is composed of the usual simply 

 twinned feldspars and quartz, mixed with epidote, muscovite, biotite, and 

 other minerals. 



At the upper contact of the white gneiss series there are frequent tran- 

 sitions into the overlying albite-schists (Hoosac schist); the transition is 

 caused by the appearance of bands of mica in the white gneiss alternating 

 with 1 muds of feldspar. The latter are often lenticular and composed of the 

 simply twinned feldspars which in the schist are proved to be albite. 



&3S3PJ&> -:-■ 



Fig. 18. — Metamorphie conglomerate (Vermont formation). From dump of Central shaft. About one-fourth natural size. 

 This is also the typical conglomerate. The pebbles are mostly of the finegrained granulite type. The finegrained 

 layers, of which a good example is seen near the top. are composed of quartz grains, biotite, and some feldspar. They 

 represent, of course, sand layers in the original sediment which have undergone considerable ruetamorphism. 



The last and perhaps the most important member of the white gneiss 

 series is the metamorphie conglomerate. This rock occupies a large area 

 in the tunnel, occurring on both sides of the central core of granitoid gneiss. 

 Nearly all the varieties of the rock are well shown by the dumps of the 

 central and west shafts of the tunnel. On the surface it is found on the 

 crest of the mountain in the line of the axis of the fold, where the rocks 

 have a gentle northerly dip, and measured between conformable contacts 

 with granitoid gneiss below and schist above, it has a thickness of about 

 650 feet. 



