210 THE PHENOMENA OF DRIFT, OR 



excavation has been carried on so far, that the wall of slate, on its 

 east side, is in some places as much as twenty feet high. Yet 

 towards the north end, the slate has not been excavated so far 

 towards the east ; so that as one stands in the south part of the 

 quarry and looks northerly, he sees a mass of the slate projecting 

 beyond the wall on his right, as is shown in PI. VIII, f. 16, which 

 was sketched from the south end. The upper part of the portion 

 which thus projects, to the depth often or twelve feet from the top, 

 is fractured and bent westerly, at all angles, even from ninety 

 degrees to horizontality. To give this position to the broken 

 laminae, the force must have acted in a westerly direction, and 

 considerably downward, as the top of the hiU has not been 

 affected. 



That this fracture does not extend through the whole hill, is 

 evident by examining its top and eastern part. Its western slope, 

 above the quany, is, indeed, for several rods, covered by trees, and 

 a few feet of soil. But if we pass over the top a little, the slate 

 comes into view, having the same strike and dip as at the quarry ; 

 so that a section crossing the hill at right angles to the strike of 

 the laminae, will present an appearance as in PI. VlII, f. 17. At 

 B, and C, the rock is concealed by soil, but visible at A, as weU 

 as along the fracture. 



When I gave an account of this singular case in my first report 

 on the Geology of Massachusetts, I was not able to suggest any 

 theory as to the manner in which it could have been produced ; 

 so evident was it that the force must have acted at the surface 

 and not from beneath. Subsequent reflection and more acquain- 

 tance with the enormous power of ice on the surface, and espe- 

 cially the discovery of the case just detailed in IVIiddlefield, have 

 satisfied me that the expansion of a vast mass of ice, resting on 

 this spot, or its movement by any other agency, might have pro- 

 duced the fracture. The direction in which the force must have 

 acted here, however, is nearly at right angles to that taken by the 

 drift in that part of the country ; so that if ice were the agent, it 

 must there have made a lateral movement ; perhaps in conse- 

 quence of an enormous mass striking against the hill east of the 

 quarry. 



