156 



an abrupt change of dip ; the slate dips steeply to the west, 

 while the gneiss has gentle eastward dips. This may be 

 considered as proving the existence of a fault on the west side 

 of the synclinal. The downthrow was to the east ; and the 

 high range of hills, of which Wachusett forms the culminating 

 point, is possibly a remnant of the grand escarpment produced 

 by this dislocation of the strata, marking the point where the 

 downthrow was greatest. This great stratigraphic break is 

 clearly indicated in the east-west section across the central 

 portion of the State, forming part of plate 54 in Prof. Hitch- 

 cock's Final Report on the Geology of Massachusetts. 



The common boundary of the first band of mica slate and the 

 narrow band of granite and granitoid gneiss, on the east side of 

 the valley, marks the position of a second great fault, parallel 

 with that just described. Here, also, the downthrow was to the 

 east, and has been sufficient to cut off very obliquely a great 

 thickness (4,000 or 5,000 feet) of mica slate and bring the 

 underlying gneiss up to the present plane of denudation. The 

 lateral displacement produced by this fault has not been less 

 than a mile, being measured by the distance between the cen- 

 tres of the first and second bands of mica slate ; these two bands 

 having, prior to the fracture, formed a single continuous stratum, 

 which, as already stated, is the stratigraphical equivalent of the 

 third band. The apparent thickness of the mica slate is less 

 than one-third as great on the east side of the argillite as on 

 the west. This enormous expansion may be due to the inac- 

 curate determination of boundaries, since the argillite, on the 

 west especially, passes rather insensibly into the mica slate ; 

 or it may result from subordinate faults or folds, yet unde- 

 tected, involving the mica slate alone. The second fault prob- 

 ably extends as far south as Worcester. Northward it cuts 

 the rocks a little obliquely, so that near the southern line of 

 Bolton we find it leaving the western boundary of the first band 

 of mica slate and passing to the westward of the attenuated 

 southern end of the Harvard and Bolton belt of argillite and 

 conglomerate. 



