REASON FOR VARIATION IN THE CONGLOMERATE. 215 



On the dump of the central shaft of Hoosac tunnel lies material excavated 

 from the eastern side of the granitoid gneiss core, and from the overlying 

 Cambrian conglomerate, 1,200 feet below the surface. The granitoid gneiss 

 in these large fragments is highly foliated, though coarse. Its large crystals 

 of microcline are still present as elongated augeu, many of which show cores 

 one-half to three-quarters of an inch across, of continuous but wavy cleavage, 

 while the rest of the individual is broken up into a mosaic by granulation 

 and traversed by the greenish mica and epidote of the foliation. In other 

 individuals, the granulation is almost complete. The blue quartz is also 

 largely broken up by granulation, and the whole rock tends towards an 

 augen-gneiss habit. 



We find great blocks of undoubted conglomerate, which simulate so 

 closely this foliated granitoid gneiss that, except for the pebbles of blue 

 quartz and occasional rock pebbles, it would be difficult to distinguish 

 between them. In these the large detrital feldspars are almost wholly gran- 

 ulated. Large numbers of other blocks of the conglomerate are composed 

 mainly of pebbles of the granitoid gneiss and detrital microcline and .blue 

 quartz with the same secondary mica that marks the foliation of the granitoid 

 gneiss in the blocks described above. 



I imagine that we have here, in the highly foliated granitoid gneiss, the 

 lower portion of the semi-disintegrated zone ; and that those blocks of con- 

 glomerate which simulate it so closely are from the immediately overlying 

 part of the Cambrian clastic beds, and made up largely of the gruess and 

 cores derived from the more disintegrated granitoid gneiss. The foliation 

 in both rocks is marked by roughly parallel courses of the micaceous con- 

 stituent, which is a secondary formation in the planes of the slipping move- 

 ment. 



I think I have thus explained the puzzling variation in character of the 

 lower Cambrian quartzite-conglomerate, white gneiss, and coarse gneiss forma- 

 tion of the Green mountain range. The transitional beds on Hoosac moun- 

 tain, under the Cambrian gneiss on the eastern and western flanks of the arch, 

 represent, I think, not only the beds formed from coarse feldspar gruess and 

 pebbles from the granitoid gneiss, but also the semi-disintegrated zone of the 

 granitoid gneiss which had escaped abrasion by the waves and which, by the 

 weakening of its cohesion, mentioned above, acted under the lateral thrust 

 to a great extent like the sediments above it. This unabraded zone of crys- 

 talline rock, which had had its rigidity weakened by beginning disintegra- 

 tion, would, under folding, pressure and metamorphisra, show on the one 

 hand a perfect and true transition into the parent crystalline rock, and on 

 the other hand pass into the much younger beds through the similarity of 

 constituents derived from it ; and an apparent conformity would be forced 

 upon the whole series, and the time-break would be masked by the foliation 

 induced by the shearing action due to slipping movement. 



