VARIATIONS IN HOOSAC MOUNTAIN CONGLOMERATE. 



213 



stratigraphical unit, though varying in habit. B, Ba, Bb, Be, in figure 3, 

 represent the variations in habit of this formation. 



Where this formation mantles over the low arch at the northern end, it is a 

 well-defined conglomerate, Ba, containing pebbles of blue and white quartz 

 and of feldspar, as well as blocks of the granitoid gneiss, up to ten inches in 

 length, in a cement of finer quartz and feldspar grains and mica, showing 

 all the marks of a clastic rock which has undergone some metamorphism. 

 This conglomerate rests here, without any transition phases, directly upon 

 the granitoid gneiss. 



Along the eastern side of the anticlinal, it is a gneiss with parallel folia- 

 tion in straight lines, marked by biotite and muscovite, Bb. On the Avestern 



W, 



E. 



Figure S—Plan of Hoosac Mountain. 



C= Hoosae schists; ^ = Quartzite; £a = Conglomerate ; £6 = Coarsely foliated gneiss (dynamic 

 product of the conglomerate) ; Be = White gneiss (dynamic product of the conglomerate); A — 

 Granitoid gneiss (pre-Cambrian). The arrow indicates the axis of the fold, pitching north. 



side, Be, along the overturned fold, it is a more massive, fine-grained, white 

 gneiss, with relatively little mica and a more obscure foliation. These 

 gneisses on the east and west seem to pass through coarse gneisses downward 

 into the granitoid gneiss. We were able at several points to trace the lateral 

 transition of the Lower Cambrian quartzite of the valley into these white 

 gneisses, definitely settling the Cambrian age of this conglomerate-gneiss 

 formation. But it seemed impossible to explain the sudden change from 

 plain quartzite to highly feldspathic gneisses, which in one limb of the same 

 arched strata are coarse, and in the opposite limb are finer with less folia- 

 tion, while the arch itself is conglomerate, and which, on the eastern and 

 western sides, pass by transitional beds, a, b, downward into the granitoid 



