82 



Petrosilex in West Roxbury, Hyde Park, Milton, and 

 the Blue Hill Region. — In passing from the coarse granite 

 of Bellevue Hill, in West Roxbury, southerly to the petrosilex, 

 along almost any line, the observer will traverse a zone of el- 

 vanite of greater or less width. It consists of a grayish- white 

 base, holding inconspicuous crystals of feldspar, and very prom- 

 inent grains of crystalline quartz. The texture is variable, 

 becoming more crystalline and coarser northerly, and less so 

 southerly ; so that this rock certainly appears to connect the 

 granite with the petrosilex. The same rock is abundant at the 

 Clarendon Hills Station, on the Boston and Providence R.R. 

 The most of the petrosilex of this area, west of the railroad, 

 besides the elvanite, is grayish or greenish, weathering white. 

 It is usually quite compact, but sometimes becomes banded or 

 porphyritic. Large masses have a quartzose aspect, appearing 

 to approach quartzite, and contain pyrite crystals. 



The last variety occurs again east of the railroad and south- 

 west of Calvary Cemetery. The pyrite at this point is 

 in minute grains, and abundant; and as a consequence the 

 rock weathers deeply and yellowish. To the east and south 

 this prominent ridge is composed almost entirely of a white or 

 whitish, more or less distinctly banded petrosilex, with here 

 and there a small irregular area or spot of purplish or red 

 petrosilex in which the banding is less distinct or wanting. 

 The banding is usually very fine, even, and continuous, and 

 very clearly results from the alternation of quartzose and feld- 

 spathic layers. 



It is at the abrupt southern end of this ridge that the 

 spherical concretions are so well developed, as noticed, ante, 

 p. 62 ; while the concretionary layers are found mainly on 

 the east side of the ridge, toward Back Street. The banding 

 is usually not apparent in the portions of the rock holding 

 spherical concretions. Between Back Street and the New York 

 and New England R.E.., the petrosilex, wherever it protrudes 

 through the covering of breccia, is mostly compact or little 



