308 GEOLOGY OF OLD HAMPSHIKE COUiJTY, MASS. 



bisects it from north to south, so that the great depression of the valley 

 seems to be connected with these ancient granitic intrusions. 



Outside the area defined above granite dikes are few and small, the only 

 large ones being the Middlefield and Coys Hill dikes. Topographically, 

 and in the interest of cartographic work, they may be divided into four 

 groups: (a) the great stocks and dikes of muscovite-granite, with their 

 accompanying swarms of smaller dikes, the whole suiTounding the groups 

 b and c below; Qi) the extended central areas of quartz-gabbro and tonalite 

 (syenite of President Hitchcock), which are without accompanying dikes; 

 (c) the biotite-muscovite-gi-anite, which occupies great areas topographically 

 as well as lithologically intermediate between the other two, and is with dif- 

 ficulty separated from the former («) because it is itself cut by an enormous 

 number of veins of muscovite-gTanite, or pegmatite, not distinguishable 

 from that of the group a itself, so that it could often as well be assigned to 

 the one as to the other on the map; (d) the porphyritic biotite-granites, 

 which are widely separated from the above group. 



At the two opposite corners of the granitic region are two great squar- 

 ish masses of quartz-hornblende rocks (tonalite), which send out no dikes, 

 and which have produced a much more intense contact metamorphism than 

 the mica-granites. 



The Hatfield tonalite area is immediately succeeded on the west by a 

 tine-grained biotite-granite almost like the Monson gneiss, but wliich from 

 the beginning carries a small, constant quantity of muscovite. To the west 

 it soon begins to be cut by jjegmatite dikes, and at the Mill River in 

 Leeds, a mile west, their number is already considerable. In the next mile 

 west tlie belt of granite which stretches from Loudville to Williamsbiu'g 

 has, as it were, a substratum of the fine-grained biotite- (or two-mica-) 

 granite, but so cut up by successive generations of the coarser muscovite- 

 granite that it almost disappears beside the latter. 



Then still farther west and south, and on much higher ground, the 

 great rounded granite stocks, which stretch from Montgomery to Conway 

 and rise to form some of the highest hills on our western horizon — 

 Pomeroy Mountain, in West Hampton, and Moores Hill, in Goshen — are 

 desolate regions of a coarse muscovite-granite, rarely slashed by great blades 

 of biotite, in which one finds here and there large areas or, as on the top of 

 Moores Hill, an isolated block of the fine-grained biotite-granite. 



