89 



less, which have all the characters of true beds.^ The grayish 

 layers become white by weathering, while the purple remain 

 unaltered. The strike varies from E.-W. to N.E.-S.W., 

 and the dip is northerly and steep. This rock, and probably, 

 also, that in Dorchester, is mainly felsite. A typical specimen 

 from the centre of the Milton area afforded only 66.3 per cent, 

 of silica. These are the most clearly and indubitably stratified 

 of all our petrosilicious rocks ; and to deny that they are strati- 

 fied is to throw doubt on the sedimentary origin of even such 

 rocks as sandstone and slate. Yet we cannot question for a 

 moment that they are essentially one with the great body of the 

 petrosilex. 



At the eastern end of this Milton area portions of the felsite, 

 where it crops through the conglomerate, pass, apparently by 

 decomposition, into a soft, greenish, unctuous rock, which most 

 observers have mistaken for serpentine ; but which appears to be 

 aluminous, rather than magnesian, and is probably a variety of 

 pinite, as suggested to me by Dr. T. Sterry Hunt. This min- 

 eral, although appearing to originate in the petrosilex, is of , very 

 common occurrence as a pebble-forming material in the Pri- 

 mordial conglomerate of this region, and a more detailed account 

 of its composition and petrological relations will be deferred 

 until that formation is reached. 



Not the least interesting of our petrosilicious rocks is that so 

 extensively developed in the southern half of the Blue Hill 

 region. This rock, which is properly an elvanite, consists of a 

 dull brown or gray base, holding numerous crystals of feldspar 

 and quartz. Its texture and composition are remarkably uni- 

 form, agreeing in this respect with the elvanite in Needham. 

 It is more crystalline, more granitic, than the Needham rock, 

 or, in fact, than any other petrosilex in Massachusetts ; and 

 this character becomes more marked northerly. The northern 

 and eastern boundaries of this area are difficult to determine 

 with accuracy, partly on account of the drift, but mainly 

 because there appears to be no sharp line of demarcation be- 

 tween the elvanite and granite ; but a gradual transition from 



'■ The lamination-cleavage is often strongly marked. 



