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. 



I.b. 



II. b. 



Quartz 



Orthoclase 



Albite molecule 



Anorthite molecule 



Biotite 



Magnetite 



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 



nib. 



iEgerine . 

 Diopside 



100.0 



20.7 



17 

 50.0 



8.8 

 2.7 



100.0 



Norms 



i. a. 



Il.a. 



Quartz 



Ort hoclase . . 



Albite 



Anorthite. . . 

 Corundum . . 

 Hypersthene 

 Magnetite. . 

 Ilmenite. . . 



Water 



CO, 



33.18 



17.24 



38.25 



3.34 



1.43 

 3.15 

 2.09 

 0.61 

 0.46 

 0.15 



99.90 



30.42 

 17.24 

 39.82 

 3.34 

 2.86 

 2.33 

 2.32 

 0.61 

 0.08 

 0.19 



Acmite 



Wollastonite 

 Enstatite . . . 



Ill.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). 

 Ill.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 

 is exposed a deep purple volcanic with a conspicuously porphyritic structure. 

 The rock is very feldspathic; the phenocrysts are white, broadly lath-shaped 

 feldspars, 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 AbgAni is the predominating feldspar, although that orthoclase is 

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

 shaped and rudely hexagonal aggregates of magnetite and secondary quartz 

 represent some ferromagnesian constituent whose substance has completely 

 disappeared. 



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



