PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS, 153 



The Sewer Tunnel, Brook Road, Milton, furnishes lava with abundant and 

 conspicuous amygdules of epidote and quartz. 



CJiemical Composition. 



The following analysis of the rock was made from composite material carefull 

 collected for the purpose from the localities represented by the slid* For 



comparative examination one of Brogger's basic akerites has been tabulated with 

 the Neponset andesite. 



The akerite possesses as mineral constituents plagioclase, a little orthocla- 

 diopside, biotite, magnetite, apatite, and a very little olivine. 



SiO,. 



A1 2 0, 



Fe,0, 



FeO 



MgO. 



CaO. 



53.75 55.00 



18.37 20.81 



8.28 



3.29 

 3.83 



Na 2 0. 

 K,0.. 



H,0 + 



H,0- 



CO* trace 



TiO, 



5.63 2.33 



3.22 5.44 



7.05 5.87 



1.20 0.16 



3.34 0.55 



P,0, 



trace 



MnO trace 



Total ; . . . 100.84 100.28 



I. Apo-soda andesite, Neponset Valley. W. H. Walker 



II. Akerite, Ullernas, T. Forsberg, analyst. Brogger, ! 



Kryst., vol. XVI, p. 49, 1890 



The mineral constituents may be recalculated as follow 



Norm. 



Orthoclase 7.23 Orthoclase 



Mode. 



• 



7.4 



Albite 37.73 Albite molecule 60.9 



Nephalite 1 1.93 Anorthite molecule 15.5 



Anorthite 14.73 Diopside 1&2 



Diopside 1.11 



Hyperethene 20.43 



Water 3.34 



96.70 

 Ophiti-neponsetose. Class 2, order 6, rang 2, subrang 5. 



100.0 



In spite of the predominance of albite in the slide, the remarkably high 

 •oda content was a surprise and the accuracy of the analysis was questioned. A 

 new analysis was made but the result showed no alteration in the soda percentage. 



The analysis falls within the chemical range of the andesites, though the 

 ruling feldspar is more acid than is characteristic of the normal andesite. In 

 accordance with the proposed use of the prefix apo these volcanics, which pre 



serve the structures of the original type — a soda-andesite— while all constituent* 



more or less completely altered, may be termed apo-andesite 



Xo representative of class 2, order 6. rang 2, subrang f>, has thus far been 



