90 



the one to the other. I have observed the granite becoming finer- 

 grained toward its southern border, until it was indistinguish- 

 able from the elvanite ; and the reverse is the fact when the 

 contact is traversed in the opposite direction. And yet there is 

 no room to doubt that this rock is a true petrosilex, and not 

 merely a fine-grained granite. The evidence on this point is 

 conclusive. A recent traverse across the Blue Hill range 

 brought me in contact with just the facts necessary to place 

 beyond question the relations of the elvanite both to the granite 

 and to the other types of petrosilex. I set out in a south-west 

 direction from the station in West Quincy, with a view to find- 

 ing and studying the contact of the granite and petrosilex. 

 The granite becomes sensibly finer-grained, in the direction in- 

 dicated, for about one mile. Here I came to a line of well- 

 marked and precipitous escarpments, with a north-west and 

 south-east trend ; just such a break in the topography as would 

 lead a geologist to suspect a fault. These cliffs are a good 

 granite, but the first outcrops a few rods beyond are petrosilex ; 

 the cliffs mark the contact of the two formations, the transi- 

 tion here being very abrupt. The petrosilex is of the same 

 gray color as the granite, or a little darker, and in a compara- 

 tively small area presents several varieties of texture and com- 

 position. It is for the most part a good elvanite, the feldspar 

 crystals, however, predominating ; sometimes it is porphyritic 

 with this mineral alone, and occasionally it is quite compact. 

 It has a bedded structure in many places, with a southerly dip. 

 The banding by which the stratification is indicated is abun- 

 dantly obvious for recognition ; and yet the bands are not, in 

 general, so sharply defined as in other localities, having more 

 the appearance of normal stratification, though frequently show- 

 ing a tendency toward the schistose or ' ' flattened pebble " 

 structure. Other portions of the rock bear plain marks of 

 extravasation ; the evidence being similar to that noted on 

 Marblehead Neck. At other times the enclosure in the petro- 

 silex of pebbles and fragments of the same rock, giving rise to 

 a breccia structure, is clearly the result of internal crushing 



