The Geology of H Ingham. 37 



thus enable the workmen to get out blocks much more easily than 

 would be otherwise possible. The origin of this kind of jointing, 

 as first suggested by Professor Shaler, is now generally admitted 

 by geologists to be due to the effect of the sun's rays upon the 

 surface, leading to a permeation of more or less heat to a con- 

 siderable depth, with consequent expansion, and finally to a sepa- 

 ration of the rock into layers. 



The last kind of joints to which attention is called, and the 

 origin of which has been by far the most difficult to explain, are 

 those which are most readily observed upon all the exposed rocks 

 of this town. They may be seen in parallel lines upon their sur- 

 faces, sometimes extending for considerable distances, and often 

 intersected by other lines which are also parallel with each other. 

 These joints are approximately vertical and vary much in direc- 

 tion, which, in view of their probable origin, is an important 

 matter of consideration. 



Examination of the direction in many localities shows as 

 follows : — 



North and south. 



North by west and south by east. 



North-northwest and south-southeast. 



Northwest and southeast. 



North-northeast and south-southwest. 



Northeast and southwest. 



East and west. 



East-northeast and west-southwest. 



East-southeast and west-northwest. 



Others are found varying in direction from all these, but they 

 are not so noticeable. 



One of the best localities to observe this joint structure on an 

 extensive scale, although not in this town, will be mentioned here, 

 because it is within a short distance from its boundary and easily- 

 observed. It is on Beach Street in Cohasset, very near Sandy 

 Cove, where a large area of rock surface extends from the road- 

 side west on an upward slope, covering a space of several hundred 

 feet. The joints on this surface are particularly well-defined. 



The parallel lines under consideration may be observed on 

 almost every exposure of rock, sometimes several feet apart but 

 in other cases only a few inches. At one granite locality on 

 Whiting Street they occur so near each other in some instances 

 as to enable one to pry off pieces not over half an inch thick, 

 specimens of which may be seen in the collection of the Public 

 Library, made to illustrate the geology of the town. 



It has always been a source of great astonishment alike to 

 students and casual observers, to find that in the severance of the 

 conglomerate rocks the parts are often found divided as smoothly 

 as if a knife had cut them asunder, and that the very pebbles 

 contained in it are divided with the rest of the mass, instead of 



