

134 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



andesitic lavas has not been worked out; both are younger than the rhyolitic 



lavas. 



Finally, completing the igneous series, though of a much later age and ex- 

 hibiting no relation in composition or origin to any of the other rocks, there are 

 two systems of diabase dikes. 



PETROGRAPHY. 



Plutonic Igneous Rocks. 



Granite. — Fresh specimens of the normal granite of the batholith were ob- 

 tained from quarries at the corner of Center and Grove Streets, between Center 

 and Cottage Streets, southwest of Cottage Street, and, finally, on Cottage Street 



near Washington Street. 



The rock is rather coarse-grained and permits one to recognize in the hand 

 specimen idiomorphic pink orthoclases, which sometimes occur in crystals over 

 3 cm. in length, light green plagioclases, conspicuous allotriomorphic quartzes 

 and a dark green fibrous or sometimes granular constituent which seems to be a 



chloritized amphibole. 



The feldspars are plainly more or less epidotized and slickensided joint sur- 

 faces often show a coating of epidote. The leucocratic constituents predominate, 

 constituting about nine-tenths of the rock and giving to the granite its prevailing 



light tone. 



The constituents noted in the hand specimens appear in the slides and in 

 addition, magnetite, apatite, titanite, and allanite. Feldspar is the most abun- 

 dant component, constituting at least two-thirds of the rock; it appears in two 

 species, — orthoclase and albite, of which albite is about twice as abundant as 

 orthoclase; orthoclase, which is often perthitic, may be distinguished by a free- 

 dom from the epidotization characteristic of the plagioclase as well as by its 

 extinction angle. The extinction angles on the plagioclase are those of alhte witn 

 the composition of Ab 8 Ani; the albite is clouded with minute grains and aggre- 

 gates of grains of epidote, to which is owing the green color of the feldspar in tne 

 hand specimen; usually the epidote grains are uniformly but thinly distribute 

 through the mineral, but in some instances a marginal and presumably more aci 

 zone of the crystal is comparatively free from epidotization. 



Quartz occurs in considerable allotriomorphic areas, constituting about one- 

 fourth of the rock and is of the usual granitic character ; it is often cracked and 

 these cracks, filled with hematite globulites, give a brilliant rose color to the 

 quartz in the hand specimen. 



Chlorite, studded with magnetite grains and including lenses of epidote, 

 represents some completely altered ferromagnesian lime silicate; the form .^ n 1 

 cleavage of this constituent in some cases suggest a member of the amp jr°^ 

 group and in other cases longitudinal sections of biotite ; rarely original ampwboe 

 shows in scanty scales possessing the pleochroism of arf vedsonite ; these melano- 

 cratic constituents compose but a small fraction of the rock. 



