PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 151 



Norm. 



Quartz 17.22 



Orthoclase 3.34 



Albite 55.02 



Anorthite 13.07 



Wollastonite 0.58 



Enstatite 3.80 



Magnetite 1.62 



Umenite 0.91 



Hematite 2.72 



Water 1.19 



Mode. 



Quartz 13.8 



Orthoclase 34 



Albite 55.7 



Anorthite 7.3 



Biotite 12.O 



Diopside 5.7 



Magnetite 2. 1 



100.0 



CO 



0.47 



99.27 



Phyro-mariposose (Class 1, order 4, rang 2, subrang 5). 



Norms. 



Quartz 



Orthoclase 

 Albite 



Anorthite. . 

 Wollastonite 

 Enstatite . . . 

 Magnetite. . 

 Umenite 



Hematite . . . 



Water 



CO, 



I.a. 



17.22 



3.34 



55.02 



13.07 



0.58 



3.80 



1.62 



0.91 

 2.72 



1.19 



.47 



Il.a." 



99.27 



Nepheline 

 Ac mite . . , 



Diopside . . . 

 Wollastonite 



16.7 

 69.2 

 0.3 

 6.9 

 5.0 

 3.9 



1.7 

 2.6 



Ilia. 



Corundum. . 

 Hypersthene 



19.6 

 29.5 



41.9 

 1.7 

 0.8 

 4.1 



I.a. Phyro-mariposose (Class 1, order 4, rang 2, subrang 5). 



II. a. Umptekose (Class 2, order 5, rang 1, subrang 4). 



I II. a. Liparose (Class 1, order 4, rang 1, subrang 3). 



» H. S. Washington, Professional Paper No. 14, U. S. Geol. Survey, 1903, p. 253 



10 H. S. Washington, op. cit., p. 147. 



The Neponset rock is a more pronounced soda-trachyte than is the Essex 



County type. Anorthoclase occurs in the latter but was not determined in the 

 former. 



Andesite Lava. — At the crossing of Blue Hill Avenue and the New England Rail- 

 road both acid and basic volcanics are exposed in the railroad cut ; here the basic 

 rock occurs as a dike in the acid eruptive. It is a compact, fine-grained, dark 

 green chlorite rock, confusedly traversed by joint-planes; the weathered surface 

 and the joint-planes are ironstained, and the latter are so numerous as to render 

 it difficult to obtain a fresh fracture. 



The specimens from this locality show a nearly complete alteration of mineral 

 constituents with a preservation of texture. The alteration products are calcite, 

 chlorite, quartz, epidote, and kaolin; epidote occurs as a cloudy yellow aggregate, 

 filling the interstices of lath-shaped feldspars, the outlines of which still remain; 

 the feldspar substance is so completely replaced by calcite, chlorite, kaolin 

 scales, and quartz, as to render it impossible to determine the species. Magnetite 

 sparingly distributed and there are some remnants of iron pyrites undergoing 



alteration to limonite 



The fabric is trachytic and inconspicuously porphyntic 



