144 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



— 



i. 



ii. 



Si0 2 . 

 A1 2 3 



Fe 2 3 



FeO 



MgO. 



CaO. 

 Na 2 

 K 2 0. 



H 2 0+) 

 H 2 2 

 TiO- 

 Zr0 2 » 



75.46 

 13.18 



0.91 



0.10 

 0.95 

 6.88 

 1.09 



0.93 



72.85 

 12.92 



2.98 (and MnO) 



0.38 

 0.90 

 7.08 

 3.01 



0.65 



III. 



IV. 





99.50 



71.65 

 13.04 



2.79 

 1.80 

 trace 

 trace 

 6.30 

 3.98 



1.10 



71.40 

 14.76 

 1.68 

 0.72 

 0.55 

 0.10 

 4.79 

 5.16 



v. 



100.77 



trace 



100.66 



1.46 

 0.03 



100.62 



70.64 

 15.34 

 1.83 

 1.10 

 0.52 

 1.24 

 5.23 

 3.55 

 0.14 

 0.38 



0.90 



100.87 



I. Spherulitic soda-rhyolite, Berkeley. Analyst, C.JPalache. 



II. Apo-soda-rhyolite, Neponset Valley. Analys" " w 



W. H. Walker. Mass 



III 



granite 



Schmlek 



IV. Keratophyre, Boden's Point, Marblehead Neck. 



V. Rhyolite, northeast coast of Marblehead Neck. 



5. Washingt 

 Washingtoi 



It is apparent that the acid flows of the Neponset Valley are related to rhyo- 

 lites but differ from the normal type in their high content of soda. The pre- 

 dominant feldspar as shown by the slides is a soda-feldspar ; as has been stated 

 the slides give no clue to the character of the principal melanocratic constituents ; 

 the analysis shows that they must have been non-aluminous members of the 

 pyroxene or amphibole groups. According to the older terminology the lavas 



are quartz-keratophyres or soda-rhyolites. 



The chemical relationship to the old types of keratophyre is borne out by 

 a general resemblance in the thin sections. The trachytic alkali feldspars of the 

 groundmass, the perthitic orthoclase and the plagioclase phenocrysts, the absence 

 of apatite and the paucity of the melanocratic constituents ally them with the 

 quartz-keratophyres. 



In accordance with a usage defined elsewhere 13 the acid volcanics of Neponse 



Valley are aporhyolites. 



Norm. 



Quartz 21.00 



Orthoclase 17.79 



Albite 49.78 



Acmite 8.78 



Wollastonite 1.86 



Enstatite l.Q Q 



99L01 



Mode. 



20.7 



Quartz ' ^73 



Orthoclase ^q'q 



Albite g' # 8 



^gerine 27 



Diopside 



100.0 



This 



The rock is a phyri-kallerudose (class 1, order 4, rang 1, subrang 4). ^ 

 name means that the phenocrysts are not notable megascopically, tna 



magma is persalic, quardofelic, peralkalic, and dosodic 



B The material was tested for P 2 Ob as well as MnO but no trace 



found 



13 Bascom, op. cit., pp. 121-122. 



