GENERAL STRUCTURE AND CORRELATION. 



23 



great schist trough is seen on the map to turn sharply to the east/the evidence 

 of this same structure is preserved in several minor infoldings of schist. 



In the tunnel the rotten rock of the old open cut, and that which I 

 have described as the Buttress-core rock and as forming below it the 

 upward transition from quartzite horizon to limestone horizon, are con- 

 cealed by masonry. But from a point several hundred feet west of the 

 "west" shaft we find the Hoosac albitic schist, which extends some 1,400 

 or 1,500 feet further east till we reach its contact with the underlying con- 

 glomerate- white- gneiss (See PI. in, d). This last-mentioned rock extends 

 some 2,000 feet farther east to its contact with the pre-Cambrian coarse 

 crystalline gneiss of the Hoosac core. On both its eastern and western sides 



Flo. 7— Crumpled structure in the Hoosac schist above the " west shaft " 

 on Hoosac mountain, a, cleavage foliation ; 'b. stratification lines marked by 

 crumpled quartz layers. 



the contact planes show that the Cambrian white gneiss is overturned in a 

 flat-lying anticline. Leaving, now, the tunnel and climbing to the opening 

 of the "west" shaft on the flank of the mountain we find that the upper 

 part of the shaft is in the Buttress-core rock — quartzite-limestone transi- 

 tion rock — and that the same formation crops out upon the mountain until 

 we reach the Hoosac schists, several hundred feet higher up. Climbing 

 above this point we find the Hoosac schists, with evidence that they occupy 

 an inverted syncline. Fig. 7 shows the structure at this point on a small 

 scale. Above this the dips observed on both sides of the summit show that 

 the crest is a simple open syncline. 



The presence of the Buttress-core rock at the top of the "west" shaft 

 and its projection so far westward over the Hoosac schist of the tunnel 



