PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 157 



Pyroxene is represented only by a violet gray augite occurring in considerable 

 areas, always allotriomorphic and showing less alteration than the feldspar does. 

 Iron oxide is present in the form of titaniferous magnetite and of ilmenite both 

 of which are abundantly altered to leucoxene, an alteration making more marked 

 the cubic cleavage of the former mineral and the rhombohedral cleavage of the 

 latter mineral which displays curiously branching forms. Apatite is abundant 

 as is usual in this type of rock; olivine, biotite, and original hornblende are none 

 of them present. The secondary constituents are uralite, actinolite, chlorite, 

 epidote, albite, calcite, quartz, leucoxene, and pyrite; the first three arc alteration 

 products of the pyroxene; epidote in minute, stoutly columnar crystals, scanty 

 albite, quartz, chlorite, and calcite are alteration products of the feldspar. 



Neponset Valley Igneous Rocks. 



I. 



Normal Granite 



71.34 



13.97 



1.95 



1.00 



0.62 



1.63 

 4.83 



3.89 



0.80 



0.09 



0.43 



trace 



100.55 



II. 



Bfltholith. 



Fine-graineM 

 Granite. 



76.52 



12.30 

 0.70 

 0.56 

 0.16 

 0.31 

 5.19 

 4.58 

 0.41 

 0.11 



none 

 0.12 



trace 



100.96 



HI. 



Rhyolitic Faciei. 



IV. 



Acid Volcanic*. 



71.63 

 13.71 

 2.09 

 1.76 

 0.19 

 1.31 

 3.24 

 4.49 

 0.51 

 0.08 

 0.41 

 0.34 



trace 

 trace 

 trace 



99.76 





Aporhyolite LaTn 



72.85 

 12.92 



2.98 



0.38 

 0.90 



7.08 

 3.01 



0.65 



100.77 



v. 



VI. 



Acid Volcanica. 



(Center) Dike 



(Border) Dike 



73.72 

 13.22 

 1.48 

 1.72 

 0.66 

 0.65 

 4.52 

 2.90 

 0.36 

 0.10 

 0.15 

 0.34 

 trace 



99.82 



71.73 

 15.00 

 1.54 

 1.05 

 0.86 

 0.69 

 4.69 

 2.90 

 0.94 

 0.14 

 0.19 

 0.35 

 trace 



100.08 





VII. 



VIII. 



Trachyte 



65.41 



16.09 



3.84 



0.95 



1.51 



2.93 

 6.54 



0.57 



1.06 

 0.13 

 0.47 

 0.51 



trace 



trace 



100.01 



Intermediate. 



Acid Andesite Dike 



62.80 

 19.84 



8.70 



Relatively low in 



CaO and high in 

 MgO 



IX. 



X 



Basic Volcanica. 



Andesite Lava. 



53.75 

 18.37 



8.28 



5.63 



3.22 

 7.05 

 1.20 



3.34 



Andesite Dike 



100.84 



45.30 



16.07 

 8.26 

 2.17 

 5.58 



5.63 

 2.44 

 4.06 

 2.93 

 0.30 

 4.78 

 1.70 

 0.20 

 0.43 



99.85 





