GEOLOGICAL OUTLINE. 15 



(3) The, Monson gneiss. — The great plateau of Cambrian gneiss wliioli, 

 starting in Northfield, runs through Wendell, Shutesbury, and Pelhaiu, and 

 ends against the Belchertown tonalite, furnishes the key to tlie structure 

 of the region. It is another "horst"— a great area of ancient crystalline 

 rocks bounded by faults outside which the ground has everywhere sunk 

 away. ItSs, moreover, a region of very gentle dips, unlike the western 

 hill country. The rocks, horizontal in the center, dip slightly toward the 

 borders on the east and west. It is bounded by north-south faults on 

 either side, which extend wholly or nearly across the State. These faults 

 are lines or bands of extreme crushing, and outside them the rocks have 

 been compressed in sharp folds, as if they had been thrust against the 

 unyielding shoulders of the great "horst." The normal Monson gneiss 

 is, however, the ordinary biotite-gneiss. One of the faults mentioned 

 runs at the foot of the high grounds along the east border of the broad 

 Connecticut Valley, from Northlield south, through the notches at the 

 east foot of Mount Toby and at the east end of the Holyoke range, and 

 so on through Granby and Willjraham. As noticed above, this fault 

 forms also the eastern boundary of the Connecticut Valley "graben." The 

 other fault runs in a corresponding position, along the east border of the 

 valley of the west branch of the Swift River, through Wendell, New 

 Salem, Prescott, and Enfield. The faults are niarked by great crush- 

 ing of the rocks, by the development of curious "fault rocks"— bastard 

 granites and green \and buff hornstones— and by the cementation of the 

 crushed rocks by comby vein quartz and specular iron. Within the "horst" 

 the Monson gneiss contains a thick bed of a fine-grained actinolite-quartzite 

 or at times fine biotite-quartzite or biotite-gneiss. 



(5) The At7;isfe.— Outside these faults the upper schists are present in 

 the same series as west of the river, ^-iz: («) A feldspathic mica-schist or two- 

 mica-gneiss, at times a quartzite or quartz-conglomerate, is the equivalent of 

 the feldspathic mica-schist or Hoosac schists and the lower sericite-schist or 

 Rowe schists of the west side. This is named after the more persistent and 

 important bed of the western area, the Rowe schist, {h) A hornblende- 

 schist^the Chester amphibolite. (r) A micaceous quartzite, very generally 



