218 PROCEEDINGS OF THE AAIERICAN ACADEMY. 



this constituent was introduced from outside. In fact these areas 

 seem to represent a phase in the alteration of the original hornblende 

 which gave rise to secondary riebeckite accompanied by magnetite 

 and other minerals, in part introduced from the surrounding rock, 

 among these titanite, in cases where the amount of titanium was 

 adequate. In this connection it is interesting to note that titanite 

 having somewhat similar characteristics and accompanied by chlorite 

 and calcite, has been observed by Pirsson and Washington as a second- 

 ary mineral replacing hornblende (basaltic) in the fine grained camp- 

 tonites from the Belknap Mountains of New Hampshire.^* 



These altered hornblendes have doubtless given rise to the small 

 pits and yellowish decayed spots seen megascopically in the finished 

 surfaces of the quarry granites. 



The inclusions in the hornljlende in addition to the aegirite are: 

 fiuorite, in minute, usually anhetlral grains; black oxide specks and 

 grains, probably magnetite, scattered for the most part, but also ar- 

 ranged in curiously curved strings or bands, usually traversing only 

 parts of the crystal and then commonly on one side or about a portion 

 of the margin; larger, black grains of iron oxide; and a dark red 

 mineral, probably aenigmatite (see beyond). 



It is probably safe to say that the predominating hornblende is a 

 riebeckite. This is certainly true of the granite in the eastern part 

 of the quarry district (North Common Hill and eastward in Quincy). 

 In many slides from the granite of this district it is the only variety 

 of hornblende present. In the West Quincy district and westward in 

 Milton the riebeckite is in considerable part replaced by a hornblende 

 which is apparently a cataphorite or a closely related variety. In 

 many sections this variety is seemingly the only original hornblende 

 present, although it is difficult or impossil)le in random sections to 

 distinguish always surely between the two. What is here called 

 riebeckite has substantially the same optical properties as those 

 given for the hornlilende of the pegmatites previously described by 

 the writer and C. Palache.^^ These are: — a A c'4° to 5°; Axial plane 

 perpendicular to b, (010); Opt. — , Bisectrix acute = a; Axial angle 

 medium to large. Dispersion strong, giving rise to colored axial bars 

 (red and blue). Double refraction very low: pleochrism with low 

 or medium magnifying powers: a, deep-blue to smoky -blue or green; 

 7, very dark, smoky-green to almost black; /S, pale yellow or slightly 



14 American Journal of Science, 22, p. 503 (Nov .-Dec, 1906). 



15 loc. cit., pp. 152-3. 



