226 ^- ^*^Tö. 



Geoldo'ioa] Survey, establishes its cheinic:i] nature to he that of 

 l('l)id()}iie](iiie. The followiuo- percentages are those found ])\ liini: — 



SiO. 8(). 60 



ALO, 17- 05 



FeO 'A\. 29 



CaO trace 



]\rcrO 10. 86 



MnA 0- 70 



Na^O 5. :\9 



KgO 8. 49 



99. 88 



The angle of the optic axis is so small as to give almost an mn- 

 axial stanroseo]iic figiu-e. 



The hornblende-granite occurs very frequeritly in detached 

 masses and in leriticular forms, enclosed within hioiite-grauite of 

 a more acid composition and of a mnch later origin. The biotite in 

 such a cauqlU-up (jraiiite ])resents a peculisir feature not observed in 

 tlie normal minei-al. The mica is liere devoid of its crvstallographic 

 outlines, and the deep brown lamella^ of this mineral with ditferent 

 optical orientations are heaped together around the larger one. Thns 

 the microscopic scales of the biotite with its marginal ragged structure 

 possess the same habitus as tliose found nniversally in contact rocks, 

 and in many of the crystalline schists. Tlu^ larger crystals are inter- 

 lard, ed Avith needles of apatite. 



Tlie hornhlende has a bluish-green colour. The extinction-direc- 

 tion varies within wide latitudes, and gives nsually larger angles than 

 those ^of common species. The most distirictive feature of the horn- 

 blende of our granite is, however, its weak pleochroism, comparatively 

 faint polarization-colours, contrary to our expectation from its deep 

 bottle-green colour. The degree of absorption is normal : c]>6>a. 



