14 GEOLOGY OF OLD HAMPSHIRE COLTNTY, MASS. 



by-soutli fault, which appears so plaiuly on the map, forming the eastern 

 boundary of the valley across Northfield and Moutag-ue, is probably pro- 

 longed in the Holyoke range fissure from Mount Tom southward. From 

 a point north of Mount Toby a fault branches from the main one and 

 is continued down the east side of the valley, a series of great faults 

 running south by east at the eastern border of the valley; and much 

 the same seems true of the western side, and especially the two settings- 

 back of the valley border seem due to the two east-west faults; How 

 far the valley bottom has been depressed between these faults I can not 

 determine, but the great thickness of the red sandstone, as shown by 

 artesian wells, would indicate that the sinking must have been consid- 

 erable after, and perhaps during and before, the deposition of the Trias. 



The region is thus a great "graben" — a band of country sunk between 

 parallel faults; and the great Grreenwich-Enfield basin has, at least in part, 

 the same character, though here erosion has been the more important agent, 

 and in its northward extension into New Salem and Orange the sole agent. 



In both pre-Triassic and Triassic time the Connecticut Valley has been 

 a region of extensive faulting and the pre-Triassic faulting extends con- 

 sidei-ably east of the present bottom of the basin, especially in the Northfield 

 region. 



All the rocks of the area west of the Connecticut reappear in the 

 eastern region. The Bernardston rocks are present only in a few outcrops 

 in Northfield and farther south, while the Leyden argillite appears in the 

 south bank of the Connecticut just below the mouth of Millers River, and 

 seems to run down the valley beneath the Trias and to appear west of the 

 2)ond in the center of Leverett. It is also represented lithologically in the 

 center of the middle syncline in Monson. 



The salient featiu'es of the eastern area are — 



(i) The eruptive rocks, consisting of («) the great block of diallage- 

 granite, or tonalite, and quartz-gabbro in Belchertown and the surrounding 

 towns, around which the crystalline rocks are thrown into great confusion; 

 (J)) the block of diorite in New Salem and Prescott, which seems to have 

 produced very little confusion in the surrounding rocks; (c) The Coy's Hill 

 porphyritic granitite; ((/) the large granite areas in Leverett and Amherst. 



