464 WARREN AND POWERS DIAMOND HILL-CUMBERLAND DISTRICT 



somewhat more patchy in character, and albite is not in general developed 

 about the margins of the grains to the same extent as in the Qiiincy gran- 

 ite ;^^ albite in separate crystals about the larger feldspars is also of rare 

 occurrence. The microcline member is somewhat altered and shows, as 

 compared with the albite, a marked tendency to include microliths of 

 riebeckite and segirite, which are abundant in the feldspar, as is the case 

 in the Quincy granite and such rocks generally. Of all the minerals 

 present, the microperthite is the most nearly automorphic, the crystals 

 showing a rather strong tendency toward rectangular forms, although the 

 margins, if originally well defined, which is doubtful, have been destroyed 

 by shearing. 



Quartz, in grains comparable in size with the feldspar, may sometimes 

 be seen, but even these show very broken extinctions, and most of the 

 quartz is in the form of granular mosaics or streaks of small grains. The 

 riebeckite sometimes forms good sized, irregular crystals, but is generally 

 in smaller irregular prismoids or splinters and commonly shows a strong 

 tendency toward subparallel aggregation, doubtless a result of movement. 

 It is always of the strongly absorbing, pleochroic variety, deep blue or 

 bluish green to black for the rays near c' and h, yellow or greenish yellow 

 for the ray near a. The extinction is small (6 degrees to 8 degrees) on 

 the cleavage. In short, it appears to be the same riebeckite that has been 

 described in the Quincy granite.^* 



The pyroxene appears to be a pure aegirite and is closely associated 

 with the riebeckite, sometimes in parallel intergrowth with it, but more 

 often irregularly grown into or upon it in the form of small prismoids, 

 often with a subradial arrangement. 



A mineral having the characteristics of astrophyllite or a closely re- 

 lated species is an important and quite abundant accessor}'. It is found 

 in the form of slender plates or fibers about the riebeckite and gegirite, 

 between their grains, along their cleavages, or penetrating the crystals 

 irregularly: a small amount is found alone lying along the boundaries of 

 the feldspar and quartz grains. It sometimes forms minute clusters and 

 stellate groups. With its bright colors, yellow to orange, and marked 

 micaceous habit, it is a striking constituent of the rock. While this min- 

 eral is a rare constituent of the Quincy granite, it is here \ev\ abundant 

 and is perhaps the leading peculiarity- of this granite. 



Zircon is present, but comparatively rare, at least in a form that can be 

 positively identified. Fluorite, on the other hand, is common and may 



» C. H. Warren : Petrology of the alkali-granites and porphyries bf Quincy and the 

 Blue Hills, Massachusetts. Proc. Am. Acad. Arts and Set. vol. 49, No. 5. 1913. p. 213, 

 «*Loc. clt.. pp. 216-219. 



