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group probably surpasses the basic. From the granite in 

 northern Plymouth County to the Montalban gneiss in Concord, 

 this formation measures not less than thirty miles across the 

 strike, and the rocks, when not eruptive, appear, usually, to be 

 nearly or quite vertical ; but we get from this simply a maxi- 

 mum limit which is far in excess of the bounds of probability, 

 for there is no doubt but that the repetitions resulting from 

 folds, faults, and extravasation, have increased the apparent 

 breadth or thickness of the series at least fourfold. There 

 are, however, no bilateral repetitions of the rocks of such 

 magnitude as to render probable the existence of extensive 

 folds involving the whole thickness of the formation. On the 

 contrary, the more important repetitions, so far as I have ob- 

 served, are such as could be produced by faults alone. 



Summarizing what has been said concerning the distribution 

 of these rocks, we find, proceeding from south-east to north- 

 west, the following general geographical arrangement. In 

 northern Plymouth and southern Norfolk Counties there is 

 an immense mass of granite covering hardly less than two 

 hundred square miles. It is elongated transversely to our 

 line of section ; and, although largely exotic, it must be re- 

 garded, when viewed as a whole, as essentially in its normal 

 position with relation to the remainder of the formation. Suc- 

 ceeding the granite, disregarding the uncrystalline rocks under- 

 lying Boston and its environs, we appear to have a broad belt 

 of petrosilicious rocks extending to the northern boundary of 

 the Natick and Marblehead range ; and following this is a yet 

 broader belt of diorites and stratified rocks reaching to the 

 Montalban border. This generalized geographical arrangement 

 corresponds with the stratigraphical succession. 



Now it is an interesting fact that, in the northern half of 

 Essex County, we find substantially the same geographical 

 arrangement and corresponding stratigraphical succession, 

 but on a smaller scale. There, as farther south, the stratified 

 crystallines all dip to the north-west ; and we have, on the 

 south-east, the granite of Essex, Ipswich, Rowley, etc., 



