PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 133 



granite and in part of rhyolite porphyry; these two types, grading into each 

 other and into the normal granite, which they are clearly seen to cover, are alike 

 in the absence of all fluidal, spherulitic, or brecciated textures. 



Cutting both granite and peripheral zones is a remarkable dike, 100 feet 

 wide, which is alternately rhyolite porphyry and granite porphyry; the por- 

 phyritic character, which is always conspicuous, makes the dike easily recogniz- 

 able; it is one of a series and the only one of which detailed study was made; it is 

 essentially identical in composition with the granite and the peculiar textural 

 variations which it shows maybe owing to varying depth of solidification. It is 

 younger than the rhyolite porphyry which it penetrates and older than the rhyo- 

 lite dikes which are found cutting it. 



Also cutting the normal granite and both types of the contact zones, are very 

 numerous and mostly irregular dikes of rhyolite, which, varying in width from 

 an inch or less to fifty feet or more, are usually, but obscurely, porphyritic and 

 often show fluxion lines especially near their margins; this fluidal banding, always 

 parallel to the bounding walls, in one dike, three to four feet wide and perfectly 

 banded throughout, is in strict conformity to highly irregular walls of the coarse 

 normal granite. It is believed that they are contemporaneous with the acid 

 volcanics to which some of them may well enough have been channels of supply, 

 an hypothesis confirmed by a petrographic and chemical comparison. Both the 

 volcanics and the dikes are clearly much younger than the granite batholite and 

 its contact zone, which they overlie and penetrate; the cover of the batholite, 

 Cambrian slate, must have been completely eroded before the lavas were effused. 



Professor Crosby believes that he has located three necks or vents. The 

 largest and least questionable of the three occupies an area, bounded by Grove, 

 Center, Stimson, and Washington streets in West Roxbury, roughly oval in form, 

 and some 1500 by 3500 feet in dimensions. The mass, consisting chiefly of 

 brecciated rhyolite and a few included riders of slate, is sharply defined from the 

 surrounding microgranite and normal granite, penetrating which are radiating 

 rhyolitic dikes, and above which scattered in irregular patches are the remnants 

 of rhyolite flows. These lavas exhibit unmistakably effusive characteristics and 

 may always be distinguished by them from the rhyolite facies of the batholite. 

 Two other necks— Bold Knob and Grew's Wood necks— are located in Hyde Park. 



Of more recent age than the rhyolitic lavas and dikes are andesitic dikes 

 which bear a strong resemblance to the andesitic flows also exposed in this 

 district, and which, with the rhyolitic flows, are the subject of an earlier investi- 

 gation. 1 These dikes, which are irregular and range in width from an inch or 

 less to twenty feet or more, are thought to be related to the andesitic flows, which 

 are interbedded with carboniferous conglomerate, in the same way that the rhyo- 

 lite dikes are related to the rhyolitic flows. 



Among the volcanics definitely known to be of Carboniferous age are patches 

 of lava of a trachytic composition; the relation in age of these lavas and the 



1 Bascom, op. cit., pp. 122-125. 



