PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 137 



what faint pleochroism — a = bluish green, b = pale blue, r = pale greenish 

 yellow, a > b > r with extinction 14° — , may also be present. In the fine-grained 

 material the prevailing fabric is micro-graphic ; quartz and plagioclase feldspar are 

 intergrown or quartz and perthitic orthoclase. In the microgram tic material the 

 typical fabric is the orthophyric though, where perthite is the prevailing feldspar, 

 there is a tendency toward the porphyritic fabric. 



Chemical Composition. 



The material for analysis was made up from the same fresh material which 

 furnished the slides. 



Fine-Grained Granite or Micro-Granite of Neponset Valley. 



Norm. Mode. 



SiO, 76.52 Quartz 30.48 Quartz 32.2 



Al,Oi 12.30 Orthoclase 27.24 Orthoclase 27.3 



Fe,0, 0.70 Albite 37.73 Albite (Ab It An,) 9.5 



FeO 0.56 Acmite 1.85 Albite molecule 2. r >.3 



MgO 0.16 Sodium metasilicate 0.98 Anorthite molecule 1.6 



CaO 0.31 Diopside 1.43 Arfvedsonite 3.7 



Na,0 5.19 Hypersthene 0.60 Magnetite .3 



K,0 4.58 Ilmenite 0.15 



H,0+ 0.41 



H,0- 0.11 



CO, 



TiO, 0.12 



P,0, 



MnO _trace 



Total 100.96 



100.46 



100.0 



Wm. T. Hall, analyst 



The rock is a grani-liparose. 



The chemical relations of the fine-grained and the normal granites will be 

 discussed after the rhyolitic facies has been described. 



Rhyolite. — The extreme peripheral facies of the granite batholith — that into 

 which the micro-granite merges upward — is a massive rhyolitic porphyry ex- 

 hibiting somewhat conspicuous phenocrysts of quartz and feldspar, and in some 

 instances melanocratic phenocrysts. 



Specimens of this peripheral type were obtained from the Summit, from 

 Turtle Pond, and southwest of Turtle Pond Road, Stony Brook Reservation. 



The color of the rock is purple or dark green. It is distinguished from the 

 rhyolite that occurs in dikes and flows by a less fine-grained texture and by the 

 absence of effusive characters: the effusive rhyolite exhibits a cryptocrystalline, 

 jasper-like texture and breaks with very sharp edges; it also possesses all the 

 fabrics characteristic of acid lavas, and is associated with pyroclastics: the 

 peripheral rhyolite, while aphanitic, is scarcely cryptocrystalline and exhibits 

 neither fluxion, spherulitic nor amygdaloidal fabrics. 



The constituents are those of the granite massif. Quartz occurs abundantly 

 as phenocrysts and is crowded with fluid and solid inclusions. Feldspar is 

 represented by two species, orthoclase and oligoclase, which are present in approxi- 



