HOOSAC MOUNTAIN. 103 



at tlie base of the Hoosac series are not found in Mount Greylock or in 

 theTaconic range, at least not here. I am not prepared to say that the gran- 

 itoid gneiss itself might not be an altered sediment, instead of an eruptive 

 granite affected by dynamic metamorphism, but in such an extreme case 

 we need careful proof of the process of change, which we can not vet give. 

 This rock has perhaps rightly been called Archean by J. D. Dana, C. H. 

 Hitchcock, C. D. Walcott, and others, the proof resting on some litho- 

 logical resemblance or on unconformity with the overlying rock. It has 

 been shown in the previous pages that this evidence is unsatisfactory, for 

 the most absolute conformity exists in places, and the overlying rocks some- 

 times take on the characters of the granitoid gneiss. The altered trap dike 

 found in Stamford, which cuts the granitoid gneiss but not the quartzite, is 

 the first conclusive evidence of nonconformity. 



Another striking- fact is the uniform result produced by metamor- 

 phism in the originally dissimilar rocks. The amphibolites were primarily 

 trap rocks composed of hornblende and feldspar, and even the hornblende 

 may have been derived from augite and the rock a diabase; but this fact, 

 proved for rocks in other regions, is yet in doubt here. By the metamor- 

 phism of these eruptive rocks new feldspar, biotite, hornblende, etc., are 

 finned— of which minerals some occur with the same peculiar features 

 (feldspar) in the schists which have been formed from sediments (shales, 

 slates, etc.). In the process of metamorphism here there must have been 

 an important chemical action originating from without the rocks. 



A further unexplained condition is the vertical position of the plane 

 of lithologic change toward a gneissic character. The fossiliferous Cam- 

 brian quartzite (Vermont) of Clarksburg mountain forms a great dome, on 

 the east side of which it strikes northeast toward the crystalline rocks, and 

 within 2 miles, in Stamford, Vt., we find it partially changed to gneisses. 

 The quartzite of Cheshire preserves its character as quartzite until its strike 

 carries it east across a certain meridian (the west crest of Hoosac moun- 

 tain), then in a quarter of a mile, passing this line, it gradually changes 

 into a white gneiss by taking up feldspar and mica A mile or so north we 

 find that the ends of the little cross-crinkles in the white gneiss north of 

 Dry brook are quartzite and ordinary quartzite-conglomerate. They pass 

 into white gneiss when they strike east within a very short distance. 



