* 



136 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



and dosodic; the higher percentage of lime is associated with epidotization ; 

 this and the presence of biotite should distinguish the Neponset granite in the 



hand specimen. 



Since plagioclase feldspar predominates over orthoclase, the 



Neponset granite is of the quartz-monzonite type. 



Modes. 



Quartz 



Orthoclase 



Albite molecule — 

 Anorthite molecule 



Biotite 



Hornblende 



Arf vedsonite 



Riebeckite 



Glaucophane 



Magnetite 



Accessories 



Micro-granite . 



As has been stated the normal granite grades upward into a 



fine-grained granite. 



Specimens of this facies of the batholith were obtained on the west and east 

 sides of Washington Street north of Grove Street, on the railroad one-half mile 

 from Spring Street, in a quarry on Cottage Street near Washington Street, and 

 in the northeastern part of Stony Brook Reservation. 



The material is uniformly fine-grained merging into a microgramte ; it is 

 light colored, usually of a pinkish cast of color but may be greenish; feldspar and 

 quartz and a very subordinate melanocratic constituent, which seems to be 

 chlorite, may be distinguished in the hand specimen. 



The slides show the same constituents as those exhibited by the normal gran- 



ite, but the textures vary from the typical granolitic texture. 



Again there are 



prominently present two species of feldspar ; perthitic orthoclase, albite (Ab) and 



albite of the approximate composition Abi 2 An 



and in addition a little of a more 



basic plagioclase, oligoclase (Ab 6 Ani to Ab 6 Ani), and scanty microcline. Granular 

 epidote is frequently developed in the feldspar and when very abundant gives a 

 greenish tone to the rock; sericite is also an alteration product of the feldspar. 

 Quartz is more abundant in proportion to the feldspar than in the normal S 1 ^ 

 with a ratio of 33 to 63, and is often full of linearly arranged inclusions both solid 

 and liquid ; the latter show a movable bubble which can readily be discerned witn 

 a high power. 



The melanocratic constituents constitute not more than 4 per cent, oi 

 rock ; scanty magnetite is in many of the slides the only melanocratic constituen 

 when found other melanocratic constituents are represented as in the n0 " na 

 granite by a green, pleochroic, faintly polarizing chlorite associated with abun an 

 magnetite; scanty blades of a mineral of the amphibole group, exhibiting a som 



t 



