PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 161 



The magmatic classification of the igneous complex of Neponset Valley show 

 that magmatic differentiation had not proceeded far when the trachytic lava* 

 were effused : with the exception of the andesite, the igneous rocks of the Neponset 

 complex are alike in being persalic and quardofelic; the range of the alkali-lime 

 ratio is a small one ; the granite, rhyolitic periphery, and the trachyte are domal- 

 kalic; the other types, the microgranite, aporhyolite lavas and dikes are peral- 

 kalic; the range of the alkali ratio is also small and in every type the soda per- 

 centage is high; the microgranite and rhyolitic periphery are sodipotassic, fhc 

 other types are dosodic with the exception of the trachyte which is persodic. 



The andesites 



the product of more prolonged differentiation and 



were 



therefore presumably effused later than the acid lavas; with a loss of free silica 

 these volcanics are perfelic and lendofelic; the lava is domalkalic and persodic 

 and the intrusive is alkalicalcic and sodipotassic. The intrusive is more basic 

 than the effusive with a reduction of silica and of the soda-potassa ratio and an 

 increase of the lime percentage; the youngest igneous rock of the province, 

 exclusive of the diabase, is also the most basic. 



The lavas of South Mountain, Pennsylvania, like the Neponset lavas represent 

 the differentiation products of a soda-rich magma; unlike the Neponset volcanics 

 the South Mountain volcanics represent the products of more extreme differentia- 

 tion and lavas of intermediate composition are absent. 



The South Mountain aporhyolite is an alaskose (class 1, order 3, rang 1, 

 subrang 3), the meta-basalts are placerose (class 2, order 4, rang 3, subrang 5), 

 and au vergnose (class 3, order 5, rang 4, subrang 3) . 



1 1 JOURN. ACAD. NAT. SCI. PHILA.. VOL. XV. 



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