PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 155 



Chemical Composition. 



The material for analysis was obtained from the localities mentioned above. 

 The analysis of the andesitic effusive is tabulated with the analysis of the intru- 



material for comparative examination 



SiO,. 

 Al,0, 



F ei O, 

 FeO. 

 MgO. 

 CaO. 



I. II. 



45.30 53.75 



16.07 18.37 



8.26 

 2.17 



8.28 



5.58 5.63 



5.63 3.22 



NajO 2.44 7.05 



4.06 1.20 



K,0 



H,0 + 2.93 1 



0.30 f 



H,0 



CO,. 



TiO,. 



P a O,. 



MnO 



3.34 



4.78 

 1.70 

 0.43 

 0.20 



trace 



Total 99.85 100.84 



I. Andesite dike, Neponset Valley. Wm. T. Hall, analyst. 



II. Soda-andesite lava, Neponset Valley. Wm. H. Walker, analyst. 



There is a general similarity between the two analyses. The only marked 

 difference lies in the reduction in the intrusive rock of the soda percentage and 

 a correlated reduction in alumina and silica. This corresponds with the differ- 

 ence in the feldspathic constituent observed in the petrographic study of the 

 two volcanics. 



The lava is characterized by a more acid plagioclase than is found in the intru- 

 sive rock; orthoclase is present in both. In neither volcanic can the melanocratic 

 constituent be determined by microscopic investigation, in both types the 

 alumina percentage is consumed by the feldspars; accordingly the melanocratic 

 constituent was calculated as a non-aluminous lime magnesia silicate. 



Norm. 



Mode. 



Orthoclase 23.91 Orthoclase 26.1 



Albite 20.96 Albite 23. 



Anorthite 20.85 Anorthite 22.7 



Wollastonite 1.97 Diopside 14.3 



Magnetite 2.78 Magnetite 13.4 



Apatite 1.01 Apatite A 



Knstatite 4.10 100.0 



Foreterite 6.86 



Hematite 6.40 



Ilmenite 3.19 



Water 3.23 



CO, 4.78 



100.04 

 Orthophyri-shoshonose. Class 2, order 4, rang 3, subrang 3. 



The chemical composition and mineral constituents are those of an andesite. 



Acid Andesite Dikes. — There occur sparingly in this area dikes of medium 



acidity. On the border of the dike the rock is very fine-grained to aphanitic, 



