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of unequal size ; the angle having its vertex in Concord being 

 more prominent than that nearer the Merrimac. Each of these 

 angles, as we have seen, is probably on the line of a fault, and the 

 magnitude of the fault in each case is probably proportioned to the 

 prominence of the angle. Connected with the Concord angle 

 is a conspicuous tongue of Huronian strata which continues the 

 north-east and south-west boundary line to Westborough, and 

 the point I wish to make here is that evidence is not wholly 

 wanting of a similar peninsula extending from Groveland into 

 Lawrence. In passing from the Montalban granite in Andover 

 to the Merrimac, I crossed nearly vertical strata of stratified 

 diorite identical with the diorite so characteristic of the Huronian 

 area ; the strike is north-east. But for the Concord and Ipswich 

 fault, the Montalban strata would extend easterly over Boxford, 

 Georgetown, Eowley, Newbury, etc. ; and, but for what may 

 be designatedas the Merrimac fracture, these newer rocks would 

 similarly spread over Salisbury to the coast. The great uplifts 

 indicated by these faults have raised large areas of Montalban 

 strata above the present plane of denudation, and given rise to 

 the en echelon form of the Huronian border. 



In the boundaries of the exotic rocks, especially the granite, 

 we have evidence that the lines of fracture are sometimes 

 exceedingly tortuous and complex. One of these demands 

 special mention. The great mass of diorite in Marblehead, 

 Swampscott, and Salem has undoubtedly been thrown down by 

 a fault so as to come geographically between the petrosilex of 

 Marblehead Neck and the Naugus Head series. The proofs 

 of this are: (1) In its present position the diorite inverts 

 the normal order as established in less disturbed regions. (2) 

 This diorite area is nearly surrounded by a belt of erup- 

 tive rocks. A continuous line of granite extends from 

 Peabody to the shore in Lynn. The Swampscott shore is 

 partially skirted by granite, and this rock appears on Ram 

 Island, Little Pig Rocks, Tinker's Island, and the southern and 

 western shores of Marblehead Neck ; so that we are warranted 

 in concluding that there is, under the water, a continuous belt 



