GENERAL STRUCTURE AND CORRELATION. 19 



the zone of lateral Transition of limestone to Hoosac schist was a zone of 

 weakness which had much to do with the overfolding along the west base 



of Hoosac mountain. These anticlinal axes are inclined gently to the 

 north. About a mile south of the tunnel, at the •■Buttress." the core of one 

 is visible as a hard, white gneiss, but at the tunnel line it has sunken to 

 where the erosion surface cuts the beds representing the lateral transition 

 of limestone to schist, where they mantle around the pitching anticline. 

 and before thev disappear under the younger schist beds which stretch 

 out from the mountain. 



While the equivalence of the Greylock column with a large part of 

 the Hoosac column can be thus asserted. I am not yet in a position to make 

 a correlation reaching into details. It is not possible with our present data 

 to subdivide the Hoosac column into equivalents of the two schists and two 

 lime>tone horizons of Greylock. There is, indeed, in the eastern half of 

 the Hoosac mountains a rather sharply defined plane of division, separating 

 the feldspathic schist on the west from the practically nonfeldspathic schists 

 on the east, and these latter are distinguished further by the fact that their 

 quartz is distributed in thin, even layers, instead of occupying lenses, as in 

 the rocks to the west. This plane is used by Prof. Emerson as the base 

 of his lower hydromica-schist, ami firms an important horizon of reference 

 in his work east of the mountain. The thickness of the albitic schi>t> 

 between this plane ami the conglomerate has not yet been determined, 

 as the structure is masked by the cleavage. It is certainly not mure than 

 5,600 feet, and probably nut less than 2.500 feet. If there are no faults 

 or foldings, it is probably about 4.000 feet. We are equally ignorant of 

 the r.-al thickness of the Greylock beds, after allowing for the effect "f 

 lateral pressure and increasing local thickness. But it is quite possible, 

 if m it probable, that these nonfeldspathic schists belong wholly above the 

 Greylock rocks. In the study of Greylock mountain Mr. Dale, by patient 

 search fur the traces of the original stratification, which have here and there 

 escaped the general obliteration caused by cleavage, has been able to work 

 out the details of surface structure quite closely, and to obtain a general 

 idea at least as to the maximum thickness of the two limestones and two 

 schists. But the compressed foldings have so altered the thickness of the 



