79 



observations ; in an area where the student should traverse 

 nearly every square rod, there are almost whole square miles 

 which I have not seen. The insufficiency of my observations 

 has necessitated the foregoing summary treatment of this 

 interesting region. Any other course would have involved me 

 in serious errors of commission as well as omission. 



There are petrosilicious rocks in Reading and Woburn, inter- 

 stratified with quartzite, hornblendic gneiss, and other rocks ; 

 but since the amount is small and the close relations to the 

 associated rocks very evident, I will defer further mention of 

 them until that group is taken up. 



Petrosilex in JSFeedham,. — Outcrops are very rare in the 

 narrow strip marked as petrosilex south of the Boston and 

 Albany Railroad, in the northern part of Needham, and hence 

 this area is largely conjectural. Near the station in Wellesley 

 the rock is reddish-brown, compact, and has a quartzose 

 appearance. It is probably continuous with the quartzite in 

 Natick, and appears to be everywhere closely associated with 

 the coarse granite. 



Quite distinct from this is the petrosilex of the large area in 

 the central and southern portions of Needham. The rock is 

 remarkably uniform over this entire area. It always presents 

 a compact, grayish or greenish- white base,porphyritic with feld- 

 spar crystals, and the most of the rock is elvanite, holding 

 grains of transparent quartz in addition to the crystalline 

 feldspar. The quartz grains are half a line to a line in diame- 

 ter, more conspicuous than the feldspar, and they seldom 

 assume the shape of crystals. Toward Newton Upper Falls, 

 and east of the Charles River, in Newton, the visible quartz is 

 wanting. So far as observed, the rock never holds pebbles 

 nor exhibits any traces of bedding ; and yet very commonly it 

 presents a slaty appearance and yields to the knife, raising 

 doubts as to its Huronian age. The base is sometimes absent, 

 or nearly so, the rock being crystalline throughout, and ap- 

 proaching granite. A specimen from the railroad, one mile 

 south of Needham Station, afforded 75.45 per cent, of silica, 



