140 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



They all belong to class 1, order 4, i. e., they are persalic and feldspar is dom- 

 inant over quartz; the batholith and the extreme peripheral facies are alike 

 in rang (2), i. e., the alkalies are dominant in relation to the lime, and are sepa- 

 rated only in the subrang; the batholith, with dominant soda, falls into subrang 

 4 and the rhyolite in subrang 3 ; the microgranite, distinguished by a lower per- 

 centage of lime, falls into rang 1, and like the rhyolite into subrang 3. 



The variation in mineral composition of the three types accords with the 

 chemical variation. The microgranite contains the highest percentage of quartz 

 of the three facies, the lowest percentage of melanocratic constituents, the 

 highest percentage of orthoclase and the most acid plagioclase. The rhyolite 

 facies shows the next highest percentage of quartz and of orthoclase. It also 

 shows the highest percentage of the basic constituents and the most basic plagio- 

 clase. The normal granite shows the lowest percentage of quartz and of ortho- 

 clase, a plagioclase of basicity intermediate between that of the microgranite 

 and the rhyolite, and a percentage of the melanocratic constituents also inter- 

 mediate between these types. 



It is evident that the peripheral facies are more acid than the central body 

 of the batholith and that this acidity does not increase continuously from center 

 to periphery: the extreme peripheral border — the rhyolite — is more basic than 

 the intermediate zone of fine grained granite. Magmatic differentiation by 

 specific gravity and convection currents may have generated a more acid pe- 

 riphery and within this periphery differentiation by fractional crystallization has 

 produced a more basic outer zone; crystallization, inaugurated here because of 

 earlier cooling and following the general law of decreasing basicity, by means of 

 diffusion would cause the residual zone of the periphery to become more acid than 

 either the outer zone of the periphery or the granitic batholith. • 



The close resemblance in composition between the microgranite and the 

 Quincy granite is suggestive: the microgranite is obviously the product of differ- 

 entiation of a magma after intrusion (secondary differentiation) ; why may not 

 the Quincy granite represent the product of differentiation of a similar magma 

 prior to intrusion (primary differentiation)? Neither secondary nor primary 

 differentiation have been carried far but since time is a factor in the process, tne 

 Quincy granite must under this view of its origin be younger than the Neponset 

 granite. 7 



Intrusive and Effusive Igneous Rocks. 



Granite Porphyry and Rhyolite Porphyry Dikes .—These dikes, traversing 

 the granite are 75 to 100 feet in width; cut by rhyolite dikes, they are post- 

 granitic and pre-rhyolitic in age. The best example of the type is a dike 100 fee 

 wide exposed on Bearberry Hill, Stony Brook Reservation : the gray groundmass 



e ached 



Massachusett 



Q 



of the 



XXXII 



e Quincy 

 17-32. 



