504 W. N. BENSON. 



and refractive index. At other times the refractive index 

 may be lower; this is probably secondary mica and chlorite. 

 Sometimes small crystals of calcite and brightly refracting 

 colourless plates (talc ?) may also be present, h'elspur t Ims 

 altered has a dull green appearance. As a general rule 

 calcite, kaolin, and epidote do not form as decomposition 

 products of felspar. 



Monoclinic Pyroxenes.— Diallage, augite, diopside, 

 hypersthene and enstatite are developed in different types 

 of plutonic inclusions. In any study of a series of gahl mud 

 rocks the question of the status of diallage always arises, 

 whether it should he considered a species, or as an altera- 

 tion form of augite and diopside. 1 Judd 2 considers it to be 

 a variety only, produced by schillerisation in much the 

 same manner as the production of chatoyant plagioclase. 

 Zirkel considers its leafy nature a sufficient index of specific 

 character. The position has been briefly summarised by 

 Henderson. 3 More recently Harker has stated his disbelief 

 in the secondary origin of the schiller plates. 4 The Pundas 

 rocks show that in those types which are most altered, 

 and in which the felspar is richest in platy inclusions, the 

 platy inclusions and leafy nature of the diallage are best 

 developed; in rocks less altered, we have merely the 

 schiller inclusions developed in varying degree, and with 

 them a trace of the parting parallel to 010. In the diopsides 

 of the peridotites this is sometimes only very faintly 

 developed, if at all. Thus the gradual transition of augite 

 and diopside into a diallagic form does not seem improb- 

 able. It is, however, in those forms, in which, though 

 there is a marked development of schiller plates, there 

 is little or no sign of lamination that Vogt's explanation 

 seems most probable. 



' s »e literature cited by Teall. British Petrography. 

 . Quart, Journ. Geol. Soc, 1884, pp. 378 an.l :<su. 



