150 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



rock; it shows some alteration to chlorite and epidote and calcite but on the 

 whole is much fresher than the andesite, and is, like the porphyry just described, 

 very feldspathic. The texture is panidiomorphic and the fabric porphyritic; the 

 feldspars which constitute almost the entire rock are broadly quadratic, and in 

 many instances show a central alteration to epidote or chlorite ; repeated twinning 

 is not common and parallel extinction shows that considerable orthoclase is 

 present, though it is not the ruling feldspar; maximum equal extinction varies 

 from 12° to 16°; extinction on untwinned sections is repeatedly 19° or 20°, from 



which it is inferred that albite is again the predominating feldsp 



Aggregates 



of magnetite, chlorite, quartz, epidote and calcite as before represent the ferro- 

 magnesian constituent and suggest biotite by their forms. 



In a cut on the New England Railroad near River Street station there is 

 exposed a curiously brecciated porphyry. Aside from its brecciated appearance 

 the rock resembles the purple porphyritic volcanic from Central Avenue, Milton, 

 distinguished in the hand specimen by a lighter color; there is the same fine- 

 grained granular groundmass crowded with lath-shaped and quadratic feldspars, 

 not often showing polysynthetic twinning, but with albitic extinction angles. 



Former ferromag 



constituents are represented by areas of chlorite 



pidote, quartz, and calcite, with heavy rims of granular magnetit 



Chemical Composition. 



Fresh material for analysis was secured from a tunnel in Hyde Park which 

 exposed more of this volcanic than it had before been possible to secure. An 

 analysis of a similar rock (keratophyr) from the petrographical province of Essex 

 County is tabulated with the Neponset Valley volcanic. 



Si0 2 . 



Al»O s 



Fe,0, 



FeO. 



MgO. 



CaO. 



Na 2 



K 2 0. 



H 2 + 

 H 2 0- 

 COt... 



Ti0 2 . 

 P 2 6 . 



MnO 



S 



Total 



I. 



65.41 



16.09 



3.84 



0.95 



1.51 

 2.93 



6.54 



0.57 



1.06 



0.13 



0.47 



0.51 



less than 



0.10 



trace 



100.01 



III. 



70.23 



15.00 



1.99 



0.38 



0.33 



4.98 



4.99 

 0.91 



1.28 



• • 



0.03 

 0.06 



0.24 



100.42 



I. Apo-soda : trachite, Hyde Park Tunnel. Wm. T. Hall analyst 

 Fennia, xW?p. lf^> Vm » iek > K ° la > ^^^Teterson, analyst, W. Ramse; 



Com^Zoof T^iVr 6 ' ^J^iS* 11 ?' Ma rt>lehead Neck. T. Chatard, analyst, Sears Bull. Mus 

 P 293, mi[ V ° L XVI ' P> 170 ' 1890; &h0 Bul1 - 14 «> U. S. G. S., p. 78, 1897; also Jour. Geol., vol. VII 



» 



