PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 153 



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

 conspicuous amygdules of epidote and quartz. 



Chemical Composition . 



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

 collected for the purpose from the localities represented by the slides. 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 orthoclase, 

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



Si0 2 53.75 55.00 



Al 2 Os 18.37 20.81 



Fe 2 O s 

 FeO 

 MgO. 

 CaO. 



Na 2 



8.28 



3.29 

 3.83 



5.63 2.33 



3.22 5.44 



7.05 5.87 



1.20 0.16 



K 2 



H%±} 3.34 0.55 



C0 2 trace 



Ti0 2 



P*0 8 



MnO 



trace 

 trace 



Total ; . . .100.84 100.28 



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



II. Akerite, Ullernas, T. Forsberg, analyst. Brogger, Zeitsch. Kryst., vol. XVI, p. 49, 1890. 



The mineral constituents may be recalculated as follows 



Norm. 



Mode. 



100.0 



Orthoclase 7.23 Orthoclase 7.4 



Albite 37.73 Albite molecule 60.9 



Nephalite 11.93 Anorthite molecule 15.5 



Anorthite 14.73 Diopside 16.2 



Diopside 1.11 



Hypersthene 20.43 



Water 3.34 



96.70 

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



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

 soda 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 constituents 

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



No representative of class 2, order 6, rang 2, subrang 5, has thus far been 



