22 GREEN MOUNTAINS IN MASSACHUSETTS. 



quartzite, while in the smaller mountain to the west, which has a grani- 

 toid gneiss core, this core is pushed up in the form of an overturned anti- 

 cline upon which the quartzite lies, in normal position on the east, while on 

 the west the granitoid is underlain, in inverted order, by the quartzite and 

 the limestone. 



A careful study of the western flank of Hoosac mountain shows that 

 its structure is not that of a simple, great, overturned fold. It consists of a 

 series of parallel, crumpled folds, one or more of which have a greater depth 

 than the others. All of them are overfolded, with their axial planes dip- 

 ping eastward and with their axes pitching about 10° northward. The 

 average chord-plane of these folds dips westward 15° to 20°, forming thus, 

 as a whole, a comparatively flat, though much crumpled, western limb of 



FIG. 6 Diagram of structure, summit of the Buttress, on west flank of Hoosac 



mountain, about one mile south of Hoosac tunnel, a, Buttress rock, upper part 

 of Cambrian white gneiss: &, Hoosac schist. The exposure at the east end is part 

 of the long 1 rough infolded along the whole front of Hoosac mountain. 



the main Green mountain anticlinal arch. This structure is shown in nu- 

 merous preserved fold-cores, and is illustrated in the section through the 

 "Buttress" (Plate in, o) and in the annexed diagram of the summit of the 

 same hill (Fig. 6). The " Buttress"— a high hill on the flank of the mountain 

 about one mile south of the tunnel— is the southerly extension of one of the 

 larger of these crumples, where the axis in rising to the south brings up the 

 harder core of Cambrian white gneiss. The structure is marked both by the 

 preserved fold-core at <(, just west of the summit (Fig. 6), and by the small 

 infolded troughs of younger schist at b on the summit and b on the western 

 flank. Further north, as at the tunnel line, where nearly the whole flank of 

 the mountain is covered by the schist, the crumpling is much greater, as 

 one would expect in this material, and is marked by the crumpled layers of 

 quartz (Fig. 7). Toward the south end of the mountain, near where the 



