22 IIIS'J'ORY OF COHASSET. 



But this was not the only catastrophe of earthquake 

 and lava. 



Another came many years, perhaps a million years later, 

 and it pushed up a different sort of lava, through cracks 

 running nearly at right angles with the porphyrite dikes. 



These are the east and west dikes of diabase. One 

 can easily feel sure that they came long after the porphy- 

 rite dikes, because a certain one of them cuts through a 

 dike of the earlier set. There may be many more 

 instances where the diabase dikes cut the others ; but 

 this one is easily seen on the north of Jerusalem Road 

 in the notch of the shore near Cold Spring, where the 

 biggest dike of the town, thirty feet thick, is exposed. 



The east and west diabase dikes are very plentiful 

 throughout the town. Wherever ledges are exposed, 

 there one is apt to see a streak of black diabase running 

 through it from east to west. 



There is one about two feet thick in the ledge at the 

 head of Depot Court near the engine house. 



Down by the Cove at the mouth of Gulf River there is 

 a diabase dike of this east and west group which coincides 

 exactly with the dam which is built there ; nature and 

 man being of the same mind. 



There is another east and west diabase dike which is 

 interesting enough to mention. It runs through Wind- 

 mill Point, and being more brittle than the granite which 

 encloses it, the waves have succeeded in tearing it all out 

 by dashing stones against it. 



Any one who has a back yard big enough to enclose a 

 granite ledge is apt to be the owner of one of these east- 

 west dikes of diabase. 



Professor Crosby has counted forty of this series of 

 diabase dikes, just along our shore from the beach at the 

 foot of Forest Avenue around to the harbor. They vary 

 in thickness from one foot to thirty feet. 



The largest is at the north side of Jerusalem Road in 



