91 - 



and reconsolidation ; and in still other cases there appear to be 

 small patches of genuine breccia. A concretionary structure 

 is common in this petrosilex, as noticed a^ite, p. 62 ; and the 

 color sometimes changes to reddish or brown. But the im- 

 portant point is that nearly all these different varieties contain 

 free silica in grains of sensible size, and exhibit frequent pas- 

 sages into the typical Blue Hill petrosilex or elvanite. Continu- 

 ing my south-west course, when about one-fourth of a mile 

 beyond the line of escarpments, I came to a well-marked and 

 beautiful elvanite, holding abundant crystals of orthoclase and 

 quartz, both finely crystalhzed, in crystals 1 to 8 mm. across, 

 averaging 3 to 4 mm. ; the orthoclase predominating. Small 

 crystals of hornblende occasionally make their appearance ; in 

 fact, it is almost precisely as if the constituent crystals of the 

 ordinary granite were slightly separated and • imbedded in a 

 petrosilicious matrix. The aspect is often that of graphic 

 granite. Tliis rock is recognized at once as essentially the 

 same with the Blue Hill elvanite ; and yet it is surprisingly near 

 the granite, requiring for its conversion into that rock, simply 

 that the compact matrix, which already forms but a small part 

 of the whole, should become crystalline. And this final step is 

 supplied by the same locality ; the rock just described becom- 

 ing in some parts crystalline throughout and passing into true 

 granite. The most of this elvanite is as destitute of stratifica- 

 tion, apparently, as the granite ; but occasionally traces of the 

 original banded structure appear, even in that which is now 

 decidedly granitic. In short, these rocks, as I read them, afford 

 evidence of a gradual and bona fide transition from banded pe- 

 trosilex to granite. This is the view of the relations of the Blue 

 Hill elvanite and granite held by President Hitchcock. (See 

 post, p. 95.) A typical specimen of the elvanite or granitoid 

 petrosilex from the principal summit of the Blue Hills yielded 

 71.84 per cent, of silica, which is, I believe, about the silica 

 ratio of the Quincy granite. 



Petrosilex in Hingham and Plymouth. — There is an 

 interval of more than six miles between the most easterly of 



