GENERAL STRUCTURE AND CORRELATION. 9 



covered, apparently conformably, by a true quartzite. At the base of the 

 Dalton hills the quartzite was found to conformably underlie the great 

 Cambro-Silurian limestone, which in its turn forms the base of Greylock, 

 and this limestone was found to be conformably overlain on Greylock by 

 a great thickness of schists, identical in character with those overlying on 

 Hoosac — here the conglomerate and there the white gneiss — with no inter- 

 vening limestone or quartzite. Again, we had found that these white 

 gneisses contained apparently interstratified beds of these same schists 



CORRELATION. 



Having made it a rule that all correlation of strata and interpretation 

 of structure should be decided solely upon observed structural relations, 

 there was nothing to be done but patiently to work out the structure, step 

 by step, using lithologic similarities as (dews only. 



The reconnaissances showed that the Green mountains are wholly 

 made up of crystalline schists, and that one or more of the horizons of 

 these must vary in the most protean manner in the external habit of its 

 rocks, while on either side of the range 1 the rocks retain their respective 

 characteristics with relatively little change. One of the earlier observa- 

 tions on the western brow of Hoosac mountain had been the superposition 

 of the coarse granitoid gneiss over the white gneiss at a well-marked con- 

 tact and with structural conformity of lamination. On the other hand, in 

 the tunnel, this same granitoid gneiss appeared as a central core, farther 

 east than the geologic meridian of the surface outcrop. This core was 

 found in the tunnel" to be flanked on each side by the conglomerate over- 

 lain bv the albitic schist. If the structure were as simple as the tunnel 

 section seemed to indicate it would point to two horizons of the granitoid 

 gneiss, and connect this rock and the white gneiss in age. An important 



■ Dana pointed out in 1872 the abrupt lateral transitions between the quartzite and schists of 

 Berkshire county. (Am. Jour. Sci., 1872, p. 368.) 



2 This tunnel is lined with masonry at irregular intervals to such an extent that a large part of 

 the rock, especially of the more interesting western half, is hidden. The walls are covered to a depth 

 of an inch with soot. In addition to this, geologic work was made extremely dangerous by the fact 

 that the smoke was so dense that even our thirteen torches were invisible across the tunnel, ami the 

 noise of traius running 30 miles an hour was not audible until the engine was within a few yards from 

 us. Notwithstanding these difficulties, we managed to find the important contacts, except at the 

 western end, where they were bricked over. 



