PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 135 



The rock texture is granolitic; pressi 

 microcline present in the material studied 



effects 



not conspicuous 



Chemical Composition, 



All the analyses of the Neponset valley igneous rocks were made in the 

 chemical laboratory of the Massachusetts Institute of Technology. Typical 

 fresh specimens of the normal granite were selected for chemical analyses from the 

 quarries represented by the slides. 



With the analyses of the Neponset granite have been tabulated for compara- 

 tive purposes two analyses of granite from adjacent districts; a hornblende- 

 granite from Essex County, and a riebeckite-granite from the Blue Hills complex. 



SiO, . . 



A1,0,. 



Fe,0, . 



FeO.. 



MgO . 



CaO.. 



Na,0 . 



K 2 0.. 



H 2 + 



HiO- 



TiO,. 



MnO. 



III. 



73.93 

 12.29 

 2.91 

 1.55 

 0.04 

 0.31 

 4.66 

 4.63 

 0.41 



• • 



0.18 

 trace 



100.55 



100.54 



100.91 



I. Arfvedsonite-granite, Stony Brook Reservation. Wm. T. Hall, analyst. 



II. Hornblende-granite, Rockport, Cape Ann. H. S. Washington, analyst. 



III. Riebeckite-granite, Hardwick Quarry, Quincy, Blue Hill. H. S. Washington, analyst 



Norms. 



Quartz 



Orthoclase . . 



Albite 



Anorthite . . 

 \Vollastonite 

 Enstatite . . . 

 Magnetite . . 

 Ilmenite . . . 

 Hematite . . . 



I. a. 



24.72 

 22.80 

 40.87 

 5.00 

 1.28 

 1.60 

 2.09 

 0.76 

 0.48 



Diopside 

 Hypersthene 



II.a.« 



35.4 

 29.5 

 32.0 

 0.8 

 0.8 

 0.2 

 0.9 

 0.5 



Ac mite 



Ill.a.* 



28.9 



27.2 



37.2 



1.8 



1.5 



0.4 



3.2 



I. Arfvedsonite-biotite-grano-lassenose. 



II. Hornblende-grano-liparose. 



III. Riebeckite-grano-liparose. 



The Neponset Valley granite differs from the adjacent Quincy granite and 

 from the Rockport granite chiefly in its higher percentage of lime; the Quincy 

 granite has no anorthite molecule, is peralkalic and sodipotassic, in which it 

 closely resembles the Rockport granite. The Neponset granite is domalcalic 



• H. S. Washington, Professional Paper No. 14, U. S. Geol. Survey, 1903, p. 144. 



