138 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



mately the same proportions and together constitute about 60 per cent, of the 

 rock ; extinctions on the phenocrysts indicate oligoclase of about the composition 

 AbsAni : sericite, chlorite and epidote are secondary products of the feldspar. The 

 groundmass is a fine-grained, granular mosaic of quartz and feldspar, in which 

 the latter mineral predominates. 



The melanocratic constituent, constituting less than 10 per cent, of the rock, 

 is represented by green blades of a pleochroic chlorite with iron oxide marking the 

 basal cleavage of the original mineral and lenses of epidote lying parallel to this 

 cleavage. Magnetite and apatite are accessory constituents, and chlorite, epidote, 

 iron oxides, sericite, and calcite are the secondary products. 



Chemical Composition, 



The material for this analysis was obtained from the localities represented by 

 the slides examined, and was of an exceptionally fresh character. 



Rhyolitic Facies of Granite, Stony Brook Reservation. 



Si0 2 . 



A1 2 3 

 Fe 2 8 



FeO. 



MgO. 



CaO. 



Na 2 



K 2 



H 2 + 9-51 



71.63 



13.71 

 2.09 

 1.76 

 0.19 

 1.31 

 , 3.24 

 , 4.49 



H 2 

 C0 2 



0.08 

 0.41 

 0.34 



trace 



Ti0 2 



pQ < 



MnO.V. small amount (less than 0.10) 



g small amount 



Total 99 - 76 



Wm. T. Hall, analyst. 



Norm. 



Mode. 



32.16 Quartz ^0.5 



Orthoclase 26.69 Orthoclase ^ 



Quartz 



rvl . , f aibite'.'.V 27 - 7 , 



Oligoclase [ anorthite 6.5 



Albite.. 27.25 



Anorthite 6.39 - i auui mi«* „ g 



Corundum 1.12 Biotite 2 



Hypersthene 1.42 Magnetite '—-^ 



Magnetite 3.02 l°° u 



Ilmenite 61 



9&66 



The rhyolite differs from both the normal granite and the fine-grained granite 

 in showing no indication of an amphibole constituent. The chloritized ierro- 

 magnesian constituent possesses the form and cleavage of biotite and is ben 

 in a manner characteristic of that mineral ; the analysis sustains these indication 

 of the character of the original mineral. 



The rhyolite is a phyro-toscanose. 





