20 HISTORY OF CO H ASSET. 



but sedimentary rock laid down by the action of water, 

 while our distinction in the geological world is the pos- 

 session of plutonic igneous rocks in such mixtures as to 

 show their typical relations to each other. 



The behavior of these kinds of rock while they were 

 hardening from the plastic stuff has been partly intimated. 

 In the course of a million years, more or less, these hard 

 rock beds that were spreading underneath all the eastern 

 part of New England, nobody knows how much farther 

 inland or out under the Atlantic, were stiff enough to 

 crack. The third kind of granite has already been 

 accused of thrusting itself up through the cracks in the 

 others ; but there was a much more serious amount of 

 cracking later on. Earthquakes were more in earnest in 

 those days, and a movement of the granite rock next to a 

 crack would sometimes shove one side up and the other 

 down so that the crystals which were broken apart by the 

 crack would become separated many feet from each other 

 without opening the crack. These cracks or faults, as 

 they are called, could be detected very easily in rocks 

 that have layers and streaks in them ; but our granite is 

 so evenly mixed and colored that one is never sure 

 whether the crystals on the opposite sides of a crack 

 were originally neighbors to each other or dwelt far apart. 

 But of this we are sure, that the cracks were as many as 

 thousands running in every direction through the granite 

 which afterwards became Cohasset, 



Some of these cracks were used long afterwards by 

 nature as vents through which the lava from beneath 

 oozed up. As the granite, all under the New England 

 coast, was being wrinkled or humped up by one of those 

 movements of the earth's crust which strain our imagina- 

 tion now as they always have strained the rock, some of 

 these cracks were opened, and filled with lava forming 

 dikes. 



Upon one occasion, perhaps millions of years after this 



