160 GEOLOGICAL AND MINERALOGICAL NOTES. 



cester, is seen as an oblong mass, twenty feet in length, 

 tapering to a point near the surface of the dome-shaped 

 granite boss. The enormous force exerted by the intru- 

 sion of the granite magma from beneath upon these Cam- 

 brian beds must have distorted them and left their entire 

 surface a series of faults, cracks and crevices, thus expos- 

 ing them to all the various forces of erosion and decay. 

 The work of the ice sheet during the glacial period must 

 necessarily have been upon these sedimentary beds scour- 

 ing and grinding them to rounded boulders and fine till 

 which were deposited all over Cape Ann and in the waters 

 of the Atlantic ocean. One of these stratified boulders 

 on Ten Pound island in Gloucester harbor, and another 

 on Thatcher's island are typical examples of the larger of 

 these fragments, while in Whale cove are great numbers of 

 these stratified boulders of all sizes and of every shape. 

 This would account for the absence of glacial grooves and 

 stria? on much of the surface of the granite areas, for 

 probably the ice sheet never touched the larger portion of 

 the granite. Aerial decay has since destroyed all that 

 was left of these sedimentary beds after the ice period, 

 except such remuants as we find to-day. The absence of 

 fossils in these remaining beds is in part due to contact 

 metamorphism, for only twenty miles away at JefTry's 

 Ledge on the east, and at Rowley on the west of this 

 granite area, we find numerous fossils to complete the 

 geological history of the Cambrian deposits. 



A large number of thin sections from all the outcrops 

 have been studied with the microscope to determine the 

 detrital character of these stratified beds. The results of 

 these examinations have invariably sustained the deter- 

 minations made in the field. 



Peabody Academy of Science, 

 /Salem, August 7, 1891. 



