120 



PETROLOGY OF THE ALKALINE ROCKS 



Microscopically, kolocrystalline, megaporphyritic (hiatal), dosemic, magnophyric. 

 Fabric of ground-mass, trachytic. The phenocrysts have the outlines of hornblende, 

 but seem under the low power to consist of magnetite, but under the high power there 

 is seen to be present, in addition to the magnetite, a colourless augite (diopside), some 

 greenish augite ( pegirine augite) and a reddish-brown mineral with hornblende cleavage 

 and straight extinction. The last mentioned has the pleochroism of kaersuetite, but 

 may be a hypersthene similar to that formed in similar pseudomorphs in kulaite.* 



Larger remains of a highly pleochroic reddish-brown to greenish-brown (basaltic) 

 hornblende constitute the nuclei of several of the pseudomorphs, and may be original. 

 It is quite possible that this is a complex amphibole which has split up in some instances 

 into an aggregate of magnetite, diopside, a>girite and a simpler titaniferous hornblende 

 (kaersuetite). 



The grains are too minute to allow one to be confident of the determination. 



That the black phenocrysts are psuedomorphs after hornblende is certain. 

 {See Plate III, fig. 5.) 



TABLE IV. ANALYSES OF CAPE BIRD ROCKS 



The trachytic base consists of soda-sanidine, »girine augite, arfvedsonite, and smaller 

 amounts of magnetite and haematite. The felspar and pyroxene are idiomorphic, the 

 arfvedsonite occurs in allotriomorphic grains, and the magnetite is mostly secondary 

 (opacite). Another slide made from a different specimen, P. 315 (626), is similar, 

 but the pseudomorphs contain larger nuclei of the original unaltered hornblende. 



* Washington, op. cit. ante, p. 119. 



f Summation here includes chlorine and other constituents estimated by Washington but not 

 incorporated in this table. 



