WARREN. — ALKALI-GRANITES AND PORPHYRIES. 207 



the porphyry in the region about Chickatawbut Hill, of one or two 

 narrow, fine grained granitic stringers cutting the granite elsewhere 

 near its contact or passage into the contact porphyry, and of the small 

 dikes of pegmatite found at one or two points in the area (also near 

 the porphyry cover), dikes, genetically connected with the alkaline 

 magma, are conspicuous by their absence. The complementary dikes 

 formed by differentiation, which are so prominent a characteristic of 

 many intrusions of alkaline rocks, are here entirely absent. This 

 peculiarity is believed to be due in part, as will be pointed out later, 

 to the chemical composition of the magma, and in part to the consoli- 

 dation of the magma relatively near the surface. 



The alkaline rocks as well as the slates have been cut by a later 

 series of basic dikes. These are for the most part hea\'ily altered 

 but appear to have been all essentially diabasic in character and 

 certainly bear no near relation to the alkaline rocks. Neither these 

 nor an older, pre-granitic series of trap dikes cutting the slates in the 

 northern part of the area will be described here. 



The entire area, like all of Eastern Massachusetts, has been exposed 

 to erosion since the close of the Appalachian revolution. This erosion 

 has removed all of the carboniferous strata and nearly all of the in- 

 vaded Cambrian or older sediments, together with a great thickness 

 of the upper portions of the alkaline rocks, particularly over the north- 

 ern and once more elevated part. Much of the original porphyry 

 cover over the southern and western part of the area remains and 

 these rocks now make up practically the whole of the Blue Hills 

 proper. 



Besides its alkaline character, somewhat peculiar chemical and 

 mineralogical characters, a leading characteristic of this intrusion is 

 that it consolidated under conditions which resulted in the formation 

 of a thick protecting cover of porphyritic or even glassy rocks, which 

 differ from those beneath chiefly in texture: diif'erentiation did not 

 take place to any great extent and erosion has left the original igneous 

 cover to a considerable extent unimpared for observation and study. 



Rock Types. — The rocks of this series are all characterized by the 

 presence of soda-potash feldspars either in the form of a homogeneous 

 mixture (true mixed crystal?), cryptoperthite or microperthite : by 

 the presence of either alkali-hornblends or pyroxenes or both, and, 

 with the exception of one member, by the presence of abundant 

 quartz. They may be divided into the following types: — 



I — (a) Medium to coarse grained, riebeckite-aegirite-micro- 

 perthite-granite (Quincy type): (b) the same, but with an incon- 

 spicuous porphyritic habit. (Rattlesnake Hill type.) 



