28 HISTORY OF CO HAS SET. 



and some chloritic minerals give the dark greenish gray 

 color to the east-west dikes. Traces of iron are to be 

 found in the diabase. In fact one of the dikes is so full of 

 magnetic iron that the compass goes crazy over it. It is 

 one of the east-west system, about nine feet thick, and is 

 to be seen for three hundred feet at its outcrop a little 

 east of the notch and spring before mentioned. 



Little cubes of iron pyrites are to be found in some, 

 easily seen with a microscope. The diabase is about one 

 seventh heavier than granite, one specimen having a 

 specific gravity of 2.964, It is so very hard that the 

 marks of a date, " 18 16," upon a loose piece of it near the 

 water at the Black Rock House, look as fresh as though 

 •cut within one year instead of eighty years ago. But it is 

 more easily destroyed than the granite because it is so 

 brittle and cleaves so readily into square blocks. The 

 eruption of these dikes of diabase is substantially the last 

 event of the history of Cohasset in the making of solid 

 rocks. 



The process of erosion had long been at work scaling 

 •off the top, and it was continued without interruption. 



There was a series of movements in the solid rock after 

 its formation that has deeply affected our present condi- 

 tion. 



It is what the geologists call "faulting," the slipping 

 up or down of adjacent masses of solid rock. Wherever a 

 steep, smooth wall of granite is to be seen, it is apt to be 

 nature's mute confession of a fault or slip. That ledge 

 may safely be charged with having been thrust upwards, 

 or the part which is gone may have been dropped down- 

 wards. 



This is not always the fair inference ; because sometimes 

 a soft streak in the rock has yielded so much more rapidly 

 to the teeth of decay that the hard part is left an abrupt 

 wall. 



The story of slipping is told sometimes by the hard 



