1922.] ROCKS OF ALDERXEY. 141 



sporadic nature of brown biotite is remarkable. It is never 

 seen in well defined outline, appearing only in shreds and 

 masses of several differently orientated fragments, suggest- 

 ing derivation from an earlier mineral which is not clearly 

 seen. The existence of surrounding areas, green in colour, 

 flecked with magnetite veins, arranged in converging lines, 

 rudely suggesting hornblende, offers as explanation a de- 

 composed amphibole. Many included areas in the biotite 

 aggregates contain fairly clear felspar, and some of the 

 plagioclase is the best formed mineral present, seeming to 

 indicate later development. Isolated sections of apatite 

 occur in biotite, but these are rare. Associated with biotite, 

 but frequently centred in orthoclase, are square sections of 

 magnetite. 



There is strong suggestion in this rock that biotite has 

 formed at the expense of hornblende. 



La QUOIRE QUARTZ DiORITE. There is sufficient evid- 

 ence that felspar crystallised late, followed closely by quartz, 

 which is interspersed. The quartz is not in large quantity, 

 and is always associated with felspar. There is great pre- 

 ponderance of clear, lath-shaped, interlacing, polysynthetic 

 felspar, extinguishing at a low angle; the twin plane trace 

 being the line of reference. The orthoclase, less common, is 

 highly kaolinised, and has less sharply defined borders. 

 Interesting decomposition effects are seen. Biotite, in fair 

 quantity, in some cases an original constituent, rather de- 

 velops magnetite than chlorite. Large chloritised masses 

 contain biotite in the centre. The abundance of chlorite 

 associated with biotite, yet giving no indication of its origin, 

 whether hornblende or biotite or other material, is note- 

 worthy. 



True amphibole is scarcely seen, although instances 

 occur of magnetite veins along hornblende cleavage direc- 

 tions, in crystals of augite. Similar cleavage infillings are 

 seen in green material developing biotite at the periphery. 

 Some biotite contains traces of true hornblende cleavage. 

 One may see chlorite, biotite, hornblende cleavage, au- 

 gite cleavage, and magnetite streaks, all in one irregular- 

 shaped fragment. Such masses may be completely sur- 

 rounded by plagioclase with a little quartz. 



Faintly tinted augite, with high refractive index, is 

 common. This mineral, with oligoclase, seems to be 

 characteristic. Hornblende is subordinate, perhaps only 

 derivative, and biotite is less common than chlorite. Apatite 

 is a common accessory, some sections being fairly large. ' 



Of uralitisation there can be no question, as basal sec- 

 tions of pale augite are seen in a larger periphery of green 



