224 PROCEEDINGS OF THE AAIEKICAX ACADEMY. 



high 0.04. In convergent hght a .single biaxial optic axis was obtained 

 on the edge of the field. According to the writer's observations the 

 elongation is more likely parallel to a than c'. It appears from its 

 mode of occurrence to be a mineral formed Ijy pneumatolytic 

 processes. 



Calcite is occasionally observed, sometimes filling small interspaces 

 between the other mineral grains, again as small patclies within the 

 feldspar, and associated with titanite etc. in the altered hornblende 

 groups. Associated with the calcite and titanite, also alone, grains 

 of a mineral which seems to be siderite has been observed. This has 

 been identified in some of the porphyries. Its presence is not sur- 

 prising in a rock where iron is so abundant and lime almost lacking. 



Special Variations of the Granite. — Four variations from the normal, 

 gray granite may be specially noted. The first is that found at the 

 Gold Leaf quarry already alluded to in the description of the recry- 

 stallization of the feldspar, p. (214), and the granulation of the quartz. 

 Macroscopically the striking feature of this variation is the finely 

 granular character of the quartz which is often stained reddish or 

 yellowish with iron oxides. Besides these stains there are numerous 

 red spots that appear in part to be due to an impregnation of small 

 feldspar grains with iron oxide, and in part to the occurrence of dis- 

 tinct grains of some red mineral. Although very difficult to ol)tain 

 satisfactory data regarding it, it seems to correspond closely to the 

 aenigmatite and is so regarded. 



The second variation is that found occurring as a rather sharply 

 defined streak crossing the Ballon Quarry on North Common Hill. 

 Its chief characteristic is its delicate purple shade of color. This is 

 due to the very general distribution of minute scales and specks of 

 hematite through the feldspar. The hornblende groups are nearly all 

 heavily altered, being changed to a mass of riebeckite shreds, magne- 

 tite and hematite accompanied by a considerable amount of fluorite 

 and calcite. The aegirite originally with the hornblende has been 

 much less effected by the alteration. This streak appears to have 

 been one in which pneumatolitic action was especially active. More 

 or less of the same changes may be noted in the regular granite of the 

 Ballou Quarry which on this account has been described by Dale^^ 

 as a dark, slightly purplish granite. 



The third variation is that known as the pinl: or red type. Its 

 distribution is quite general. As clearly pointed out by Crosby^^ 



2lloc. cit.,p. 100. 



22 loc. cit., pp. 334-8. 



