148 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



ment of feldspar laths is the predominating fabric of the groundmass. Hematite 

 is distributed as microscopic dust and in the form of trichites throughout the 

 groundmass. Aggregates of epidote and chlorite occur as decomposition and 

 hydration products of the feldspar and of the scanty biotite which, it is thought, 

 was the original melanocratic constituent; a little sericite is developed in the 

 orthoclastic feldspar. 



The gray rhyolitic dikes show the same original constituents and a spherulitic 

 fabric but have undergone, as might be expected, greater alteration. The spheru- 

 lites which are of sufficient size to be seen with the naked eye, are arranged linearly 

 and are replaced by spherical aggregates of sericite scales or less commonly by 

 quartz ; they are imbedded in a fine-grained granular aggregate of quartz and 



feldsp 



Granular epidot 



abundant throughout the groundmass and its 



formation seems to have absorbed the iron oxide which does not occur as free 

 hematite in the gray rhyolite. 



Chemical Composition. 



Samples of the dark red center of the rhyolitic dikes from three of the speci- 

 mens from which slides have been made and which represent as many different 

 localities were taken for analysis. In the same way samples for analysis from 

 three localities were taken of the greenish gray border of the rhyolitic dike. 



i. 



Si0 2 .. 



A1 2 3 . 

 Fe 2 3 . 



FeO.. 



MgO. 



CaO.. 



Na 2 0. 



K 2 0.. 



H 2 + 



H 5 0- 



C0 2 .. 



Ti0 2 . . 



P 2 5 . . 



MnO. 



73.72 

 13.22 

 1.48 

 1.72 

 0.66 

 0.65 

 4.52 

 2.90 

 0.36 

 0.10 

 0.15 

 0.34 



• • 



less than 

 0.10 



99.82 



II. 



III. 



71.73 

 15.00 

 1.54 

 1.05 

 0.86 

 0.69 

 4.69 

 2.90 

 0.94 

 0.14 

 0.19 

 0.35 



• • 



less than 

 0.10 



100.08 



72.85 

 12.92 



2.98 



0.38 

 0.90 

 7.08 



3.01 

 0.65 



no trace 



100.77 



I. Rhyolitic dike (center), Neponset Valley. Wm. T. Hall, analyst. 



II. Rhyolitic dike (border), Neponset Valley. Wm. T. Hall, analyst 



III 



Rhyolitic lava, Neponset Valley. W. H. Walker, analyst. 



There is a great similarity in chemical composition as is to be expected be- 

 tween the two portions of the dikes. The alkalies and lime are practically the 

 same. The chief difference lies in the increase in the border material of hygro- 

 scopic water, of magnesia and of alumina, and the decrease in silica and the ferric 

 oxides. The increased proportion of magnesia and of alumina and the decrease 

 of silica are very likely an original difference owing to the earlier cooling an 

 crystallization of the border, which left a slightly more acid residual center. «■ 



