256 



bcacli, wlierc the waves and frost have fully developed the 

 cleavage, the appearance is suggestive of gigantic crystals of 

 mica set on edge and undergoing exfoliation. The cleavage, 

 which, as stated, is everywhere sensibly parallel with the 

 bedding planes, is, perhaps, the most perfect to be observed 

 anywhere in the Boston Basin ; and it is a noteworthy illustra- 

 tion of the principle that when the bedding planes nearly coin- 

 cide with the normal direction of the cleavage, the latter is 

 deflected so as to make the agreement perfect, the normal 

 inclination of the cleavage in this part of the Boston Basin 

 being N. 70°-80°. The fissile character of the slate is seen 

 also in the fact that with few exceptions the numerous dikes 

 follow the bedding planes and the cleavage. 



Although we do not observe on Slate Island the minor pli- 

 cations or contortions, the strongly marked wrinklings of the 

 slate which characterize the north shore of Hingham, there is 

 abundant evidence of local stresses and deformation. The 

 indications are that during the powerful compression of the slate 

 evidenced by its high dip and perfect cleavage it experienced 

 intense but very local torsional and shearing strains, resulting 

 in the development of (1) numerous small monoclinan)endings, 

 along the axes of which the slate is often pinched to half its 

 normal thickness ; (2) innumerable parallel and overlapping or 

 en echelon, oblique rifts or cracks from an inch to a foot or 

 more in length. The rifts are invariably attended by slight 

 displacements of the layers, thus aflbrding most complete and 

 instructive examples of normal faults ; the slate, without excep- 

 tion, being bent or compressed in such a way that the hanging- 

 wall of each rift or fracture is depressed with reference to the 

 foot-wall. The displacement dies out gradually from the 

 middle toward the ends of each fault ; and the cases are frecpient 

 where monoclines can be seen passing into normal faults, a 

 simple plication changing within a few inches to an actual 

 break and slip. In many of the fault cracks, also, calcite has 

 been deposited from solution, forming miniature veins, which 



