PETROGRAPHIC PROVINCE OF NEPONSET VALLEY MASS. 159 



Baste Volcanics. 



Intermediate Trachyte. 



Quartz 



Orthoclase. . 

 Albite. .... 

 Anorthite. . 

 Wollastonite 

 Enstatite . . . 



Magnetite. . 

 Umenite 

 Hematite . . . 



Water 



CO, 



17.22 



3.34 



55.02 



13.07 



0.58 



3.80 



Andesite 



Effusives. 



Intrusiyes 



Orthoclase 

 Albite 



Anorthite 



7.23 

 37.73 

 14.73 



Orthoclase 

 Albite 



Anorthite. . 

 Wollastonite 



IX. 



1.62 

 0.91 





Nephetite 11.93 



Diopside | 1.11 |Enstatitc 



Fosterite 



Hypersthene 



20.43 



Water 



Magnetite 

 Umenite . . 



Hematite . 

 Apatite. . 

 Water . . . 

 CO, 



23.91 



2n.<.)6 

 20> 



1.97 



4.10 



6.86 

 2.78 

 3.19 

 6.40 

 1.01 



3.23 



1.78 



1 00.0-1 



45 



General Discussion and Conclusions. 



Chemical Characters, 



The range in chemical composition is not very great; the rocks of this complex 

 e, on the whole, acid and alkaline; disregarding the diabase, they range from 

 granite with a silica percentage of 76 to an andesite with a silica percentage of 

 The most acid type found in the district is the microgranite with 76.52 per 



cent, silica and the most basic type is the andesitic dike with 45.30 per cent 



The alumina percentage does not vary greatly: it is lowest in the rhyolitic 

 dike rock, 13.22 per cent., and highest in the trachyte where it is 16.09, which is 

 but a slight variation from the percentage of alumina in the andesite, 16.07. The 

 total percentage of the iron oxides is rather higher in the andesite and normal in 

 the other types. Magnesia and lime are low in the acid rocks and nearly equal 



in value; they are much higher in the andesites and are again nearly equal in 

 value. 



The alkali percentages are high in all the types and soda is constantly present 

 in greater amount than is potassa, except in the most basic type, the andesitic 

 dike, where the two alkali oxides are present in about equal amounts. 



In general the acid material is of normal acidity and the basic material of 

 normal basicity; both are rich in the alkalies with predominating soda. 



Mineralogical Characters. 



In all this igneous complex, exclusive of the later diabase, the predominating 

 feldspar is albite, as the universally high silica and soda percentages lead us to 

 nvr^nf While albite constitutes from 34 to 55 per cent, of the rocks, there is also 

 always a considerable percentage of orthoclase present. The average amount in 

 the granite is 25,5, in the rhyolitic volcanics 17.5, in the trachyte 3.4, in the 



expect. 





