450 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



down over the trap, above a small quarry, is an excellent exposure of the 

 fault showing a marked brecciatiou of the adjacent beds. 



The ridge reaches its culmination in Mount Tom because of the great 

 upthrow on the fault running at the eastern foot of the mountain, and not 

 because of any thickening of the trap sheet there; it has a thickness of 

 about 250 feet at Mount Nonotuck and about 300 at Mount Tom. It then 

 sinks down to a comparatively low level, but continues south as an 

 unbroken ridge, rising in Provens Mountain, in Agawam, to 625 feet, and 

 running, with thickness not greatly diminished, to the south line of the 

 State, and upon Percival's map of Connecticut it is prolonged without 

 interraption to the south line of Simsbury. 



Parallel to the Mount Tom fault run three others, farther south, which 

 cross the trap ridge very obliquely; and, which is of more interest and 

 importance, all four run parallel to the western rocky border of the basin. 

 One forms a gap in the range in Holyoke tlu'ough which passes the rail- 

 road which comaects this town with Westiield, and this I have called the 

 Holyoke fault. The second forms the notch for the passage of the West- 

 field River, after which I have named it. The third detennines a notch in 

 the range at the point where it enters Connecticut, and I have refen-ed to 

 it as the State-line fault.^ 



These parallel faults divide the country into narrow orographic l)locks 

 which are tilted to the east, producing the uniform easterly dip; and, further- 

 more, each block seems to be raised vertically as compared with its neighbor 

 to the east, a structure which seems most marked in the case of Mount Tom. 



This produces a pattern in the boundary of the trap ridge on the map 

 which is repeated at each fault. The western boundary of trap on sand- 

 stone below swings round in sickle shape to meet the fault, while the eastern 

 boundary of sandstone on trap is transferred to the northeast along the 

 fault line. Thus the ridges are slightly echeloned, ending in a high rounded 

 bluff on the south, while the continuation of the ridge is to be found moved 

 nortli and east and beginning in a sharp point. 



As the fault lines run so nearly parallel to the trap itself, they form the 

 boundary of the latter for long distances. This is recognizable on the east 

 by the fact that where the sandstone rests normally on the trap the upper 

 sm-face is very scoriaceous and full of inclusions ; where the fault boundary 



' See pages 370, 476, for tiurther discussion of State-line fault where it crosses the posteiior dike 

 and the river at the Holyoke dam. 



