00 



parallel cleavage cracks and straight extinction, or, in other words, sections 

 parallel to the orthopinacoid (100), also show an optic axis in convergent light. 

 The maximum extinction for longitudinal sections is about 44. The mineral 

 is therefore a colourless monoclinic pyroxene. One striking feature is the 

 very high order of the interference tints under crossed nicols. In this 

 respect, and also in the rough appearance of the surfaces, it reminds one of 

 olivine. This pyroxene occurs as inclusions in the brown hornblende. 



Coloured varieties of augite occur in the picrites of the Western Isles of 

 Scotland, and also in that of Inchcolm. In the last-mentioned rock the 

 colour of the augite frequently varies in one and the same individual. Some 

 portions are nearly colourless ; others have a marked violet tint. The latter 

 are pleochroic, and the character of the pleochroism appears to be similar 

 to that of certain augites, rich in titanium, which have been described 

 by KNOP. (D 



a and 7, yelloicish 



ft, broivn with a tinge of violet. 



The augites of the Inchcolm rock sometimes show the " hour-glass " 

 structure of Werweke. The coloured augite of the picrites usually occurs in 

 large irregular plates. It contains olivine and iron-ores as inclusions. When 

 felspar is present in forms giving lath - shaped sections this mineral 

 penetrates the augite in the manner characteristic of the ophitic olivine- 

 dolerites. Indeed there can be no doubt that many of the picrites stand in 

 close relation to the olivine-dolerites and pass into the latter in a perfectly 

 gradual manner by an increase in the amount of felspar. The picrites as a 

 group appear to stand in closer relations to the olivine-dolerites and olivine- 

 diorites than to the typical peridotites. 



Hornblende. This mineral appears to be absent, as a rule, from the typical 

 eruptive peridotites. A green hornblende (smaragdite), however, occurs 

 in the oliviue rocks which form an integral part of the crystalline schists. 

 In the picrite group hornblende plays an important part and furnishes 

 the basis for the division of this group into picrites (proper) or augite-picrite9 t 

 and the rocks for which Professor BOXNEY has proposed the term 

 /tornblende-picrite. 



The hornblende of the picrites when viewed in thin sections may be 

 brown, green or colourless. In some cases a gradual transition from the 

 brown, through the green to the colourless variety may be observed in one 

 and the same individual, and when this is the case the polarisation tints 

 under crossed nicols are seen to rise in NEWTON'S scale as the natural 

 colour of the mineral disappears. In these cases there can hardly be a doubt 

 that the colourless variety has been produced from the coloured variety by 

 a bleaching process. In other cases the boundary line between the colour- 

 less and deep brown hornblende is perfectly sharp, corresponding to a 

 definite face of the hornblende crystal, whereas the external boundaries of 



(1) Z.X., 1885, X., 58. See also ROSENBUSCH, Mik. Physiog., Band I., 1885, p. 437 and 

 p. 443. 



