LEDGES FRACTURED. 87 



We suppose the period when these glaciers existed to have been that in which the con- 

 tinent was sinking for the last time previous to the drift period beneath the ocean ; or 

 more probably when it was emerging. The amount of ice brought over the surface by a 

 northern current during this submergence, must have so lowered the temperature that 

 glaciers might have capped any summits rising even moderately above the ocean. 



Even those geologists who suppose all the drift phenomena to have resulted from enor- 

 mous glaciers, will have no difficulty in admitting the occurrence of these minor glaciers, 

 crossing the track of the principal ones, during some part of the glacial period, through 

 which all will admit the northern parts of our country have passed. 



FRACTURED AND CRUSHED LEDGES OF ROCKS. 



Nearly thirty years ago, in a Report on the Geology of Massachusetts, we described a 

 bending and breaking down of the layers of clay slate at the quarries in Ghiilford, as if by 

 an enormously heavy pressure from above, and a movement towards the west, of a 

 heavy body, as is shown on Fig. 34. This exhibits from 10 to 15 feet in the upper 

 part of Bruce's quarry, on the west slope of the hill, but extending probably to the top, 

 bent or broken down, so that the plates of slate slope at all angles from to nearly 90. 

 Their normal position is a westerly dip of about 80. The crushed edges of the slate 

 appear for a considerable distance along the east side of the valley wherever the soil has 

 been removed near the top of the hill, and the effect was evidently produced by some 

 force acting near the top of the hill and directed downward or westward into the valley. 

 So far as we can judge, the very top of the hill has not been affected, though very proba- 

 bly it was crushed originally, and that the fragments have been subsequently swept away 

 from a spot so exposed, while on the side of the hill they remain as first crushed down. 



FIG. 34 



In Professor Adams' second annual Report, he describes another case at Willard's 

 quarry, three miles northeast of Bruce's quarry. Here not only are large quantities of 

 the broken slate lying at various angles on the east face of the quarry, but behind these a 

 quantity of the layers have been bent into a zigzag form, and though loose, they still 

 retain their parallelism. This is an important fact, because it shows that the effect must 

 have been produced by a heavy pressure on the top of the hill. 



