OLBEK PERIDOTITES OF SCOTLAND. 367 



in which magnesia is the principal base, crystallizing in the rhombic 

 system — the enstatites. Both of these classes of pyroxene are 

 represented in the basic rocks of the Western Isles of Scotland ; but 

 the angites occur by far the most abundantly. 



In both the monoclinic and the rhombic pyroxenes we find a 

 series of varieties differing from one another by the proportion of 

 iron which they contain. The highly ferriferous varieties of both 

 minerals occur, however, most commonly in these rocks. 



The augites are occasionally represented by varieties which ap- 

 pear bright green by reflected, and pale green by transmitted light. 

 Such forms, which resemble diopside, occur at several points in 

 the island of Mull, especially in Glen More, and at other points 

 near Carsaig. In the island of Rum, on the flanks and summits of 

 the mountains of Halival, Haiskeval, and Tralival, this green augite 

 is remarkable for the absence of regular crystalline forms and of 

 cleavage-cracks ; it often occurs in more or less rounded and 

 irregular grains resembling coccolite. 



This green augite (diopside) of Halival, which was mistaken by 

 Jameson for pitchstone, has been analyzed by Dr. Heddle*, and his 

 analysis gives us a good idea of the composition of the least 

 ferriferous varieties of the augite of these rocks. The percentage 

 of alumina indicates that it lies on the border line between the 

 aluminous and the non-aluminous augites ; chromium does not 

 appear to have been specially sought for, but these bright green 

 augites greatly resemble the chrome-diopsides of the Iherzolites. 



Silica 50-537 



Alumina 3-348 



Ferric oxide 1-338 



Ferrous oxide 4-423 



Manganous oxide 0-230 



Lime 21-419 



Magnesia 17-050 



Potash 0-252 



Soda 0-530 



Water 0-706 



99-833 



In the great majority of these rocks, however, the augite is of a 

 black colour by reflected light, and when viewed in thin sections 

 exhibits various tints from light to dark brown. 



Although no analyses have been made of these black augites of the 

 Scottish gabbros in their unaltered condition, yet the following ana- 

 lyses of the forms which have undergone the curious redistribution 

 of their materials described in this paper will no doubt serve to show 

 the general proportions of the several bases in different varieties. 



* Trans. Roy. Soc. Edinb. vol. xxviii. (1879) p. 478. 



