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of granite reaching from the Ljnin shore to and through Marble- 

 head harbor. On the north the diorite is met by the Naugus 

 Head rocks ; and the contact of these two formations is marked, 

 as already described, by extravasated masses of the last-named 

 series, which have penetrated the Huronian diorites in every 

 direction. It is clear that this girdle of eruptive rocks, this line 

 of fire, indicates a continuous line of fracture ; and movement 

 only is needed to convert this into a fault. (3) There is an 

 abrupt lithological break between the diorite and the petrosilex 

 of Marblehead Neck and Lynn ; no indications of a transition 

 anywhere. The extravasation of the petrosilex through the 

 diorite, the evidence for which was given on a preceding page, 

 proves that the former rock underlies the latter and has been 

 carried down with it. It is probably to this great fault that we 

 owe the exposure of the Naugus Head series at this point. 



