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as to weather deeply and reddish ; and at some points the rock 

 might be properly called an iron ore. Where there is a large 

 area of granite unbroken by petrosilex, as between Ipswich 

 and Gloucester, and also to the southward of Boston Harbor, 

 there are two suppositions consistent with the view here ad- 

 vanced : the petrosilex has been swept away by denudation ; 

 or it has been all metamorphosed into granite. I think in some 

 cases the latter is the true explanation. It should be said, 

 however, that neither of the extensive areas indicated has been 

 explored with sufficient thoroughness to place the entire absence 

 of petrosilex beyond reasonable doubt. The fine-grained, nearly 

 compact eurite, into which the less hornblendic portions of the 

 granite so frequently pass, appears to be most characteristic of 

 those parts of the granite adjacent to the petrosilex ; and 

 where the granite over a wide area is devoid of petrosilex, and 

 yet embraces much of the eurite, it seems fair to conclude that 

 it comes under one of the cases supposed above, where the 

 petrosilex has been removed by erosion or completely altered. 

 Much of the granite cutting through the diorite in Marblehead 

 is almost as cryptocrystalline as petrosilex, and the elvanite 

 holding the same relation to the diorite on the shore of 

 Salem Harbor approaches very close to granite. The elvanite 

 also occurs at Hospital Point, on the Beverly shore, and in the 

 granulite at this place, as already noticed, we find a structure 

 which is very characteristic of much of the petrosilex. The 

 very crystalline petrosilex on Marblehead Neck is certainly 

 nearly as good a granite as some of the adjoining eurite. 

 Through the Lynn and Medford area of petrosilex there are 

 many instances where this rock appears to pass into eurite and 

 granite. At Wellesley the granite and petrosilex are in con- 

 tact and of the same deep-red color. In southern and eastern 

 Needham these conditions are repeated, but the color of both 

 rocks is greenish- white.' The transition here seems to be perfect, 

 at least as regards texture ; and the granite becomes gradually 

 coarser as we leave the petrosilex, passing into the coarse, typical 

 granite of Dedham and Dover. Substantially the same phe- 



