PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 149 



increase in hygroscopic water and the decrease in the iron oxides may be a dif- 

 ference owing to weathering; water has been added through the development of 

 the epidote which, it has been stated, characterizes the border material; the iron 

 oxides, which must have originally occurred in slightly increased instead of de- 

 creased percentage, have been abstracted when unappropriated by the epidote 



Modes. 



■ f 



Quartz 



Ort hoclase 



Albite molecule 



Anorthite molecule 



Biotite 



Magnetite 



Lb. 



lib. 



32.0 



17.3 



38.6 



3.4 



6.5 



2.2 



100.0 



27.3 



17.4 



40.2 

 3.4 

 9.5 

 2.2 



iEgerine . 

 Diopside 



100.0 



ui.b. 



20.7 



17.8 



50.0 



8.8 



2.7 



100.0 



Norms 



Quartz 



Orthoclase . . 

 Albite 



Anorthite. . . 

 Corundum. . 

 Hypersthene 

 Magnetite. . 



Ilmenite. . . . 



Water 



CO, 



I.a. 



33.18 

 17.24 

 38.25 

 3.34 

 1.43 

 3.15 

 2.09 

 0.61 

 0.46 

 0.15 



Il.a. 



99.90 



30.42 

 17.24 

 39.82 

 3.34 

 2.86 

 2.33 

 2.32 

 0.61 

 0.08 

 0.19 



100.21 



Acmite 



Wollastonite 

 Enstatite . . . 



I Il.a. 



21.00 



17.79 



49.78 



8.78 



1.86 



1.00 



0.65 



100.16 



I.a. Phyri-kalluderose (Class 1, order 4, rang 1, subrang 4). 

 Il.a. Phyri-kalluderose (Class 1, order 4, rang 1, subrang 4). 

 IH.a. Phyri-kalluderose (Class 1, order 4, rang 1, subrang 4). 



The resemblance between the rhyolitic dikes and flows is too close to admit 

 of any doubt of a common magmatic origin. 

 Trachyte Lava. — In the woods on the west side of Central Avenue, Milton, there 

 exposed a deep purple volcanic with a conspicuously porphyritic structure. 



The rock 

 feldsp 



very feldspathic; the phenocrysts 



white, broadly lath-shaped 



numerous and with a considerable range in size; the groundmass is 

 composed of a mosaic of feldspar grains, with little quartz. Twinning by the 

 albite and Carlsbad laws are the rule; pericline twinning is rare; extinctions on 

 010 and 001 and the maximum equal extinction angles indicate that albite ranging 



from Ab to AbgAn 



the predominating feldspar, although that orthoclase 



~~ ™ w AugAii! is me predominating leldspar, aitnougn tnat orin 

 also present is indicated by parallel extinction on some of the microliths 



shaped and rudely hexagonal 

 represent some ferromag 

 disappeared. 



Lath- 

 of magnetite and secondary quartz 

 constituent whose substance has completely 



On the east side of the avenue a ledge of breccia furnished a pebble of a similar 



