271 



This pinite is unquestionably the source of that with which 

 the slaty and schistose conglomerate overlying and surround- 

 ing the ledge of petrosilex is so replete (see page 220) . The 

 pinite pebbles are mostly quite small and well flattened ; and 

 hence considerable specimens are not easily secured. The por- 

 tion submitted to analysis was obtained from perhaps a dozen 

 pebbles from different parts of the ledge ; great care being 

 taken to prevent admixture with the slaty paste. This sam- 

 ple was analyzed by Mrs. Alice B. Crosby, with the following 

 result : — 



Si 2 59.520 



A1 2 3 21.628 



Fe O 5.840 



K 2 O 6.900 



Na 2 O .804 



H 2 O 3.490 



Cr, Mg, and Mn not determined 



98.182 



The formation of pinite by the alteration, and particularly by 

 the hydration, of feldspathic rocks and minerals, which has 

 been denied by some authorities, must apparently be conceded 

 in some cases at least. Of course, where derived from a rock 

 holding free quartz in an impalpable state, such as petrosilex, 

 the pinite, though appearing quite pure, may, as the above 

 analyses show, afford an abnormally high percentage of silica. 



(3.) Pinite seems hardly ever to be entirely wanting in the 

 Primordial conglomerate of this region, though there are com- 

 paratively few localities where, as on Kent's Island, in Newbury, 

 and Central Avenue, in Milton, it occurs in such abundance as 

 to give character to the rock, forming a distinct pinite conglom- 

 erate. It is interesting to observe, however, that these portions 

 of the conglomerate marked by a predominance of pinite debris 

 are found, with very few exceptions, in close proximity to 

 ledges of petrosilex or felsite ; and, in the exceptional cases, 

 the underlying rocks are probably petrosilicious. This associa- 

 tion is very significant. 



