224 PROF. J. W. GREGORY ON NEPHELINE-SYENITE [May I9OO, 



corner of British East Africa. It is about 18 miles west from the 

 shore, about 16 miles north of Wanga, and 37 miles south-west of 

 Mombasa. According to the Anglo-German boundary-survey it is 

 1519 feet high. 1 



The mountain is a massif of nepheline-syenite. The rock is coarse- 

 grained, noncrystalline and hypidiomorphic. In hand -specimens 

 the minerals recognizable are the large felspars, which often appear 

 twinned on the Carlsbad type, and the nepheline, which is brownish, 

 or grey tinged with 'light red'; it occurs in prisms 22 mm. long 

 and 10 mm. in diameter. The pyroxene is dark, almost black, and 

 occurs scattered through the rock in radial nests of needles, and 

 sometimes in lines bordering the nepheline. 



Examined microscopically the nepheline is seen to be usually allo- 

 triomorphic, though many idiomorphic crystals occur. The felspar 

 is determinable as anorthoclase. The pyroxene is recognizable as 

 segyrine, the groups of which often include large corroded crystals 

 of sphene. The specific gravity is 2-58. (See PI. XII, fig. 3.) 



Among the nepheline-syenites the Jombo rock resembles most 

 closely that of the Sierra da Tingua in Brazil, but the nepheline is 

 more often idiomorphic. 



Owing to the distance of Mount Jombo from the coast-line, it 

 probably occurs in the belt of the Duruma Sandstones; unless, 

 which is of course possible, the fossiliferous Jurassic shales run 

 westward up the low valley of the Umba Eiver. 



III. The Camptonitic Dykes. 



The Jombo nepheline-syenite is associated with a series of dykes 

 which traverse the surrounding sedimentary rocks. These dykes, 

 whereof Mr. Hobley has given me three specimens, belong to the 

 camptonitic group. The rock is grey, and studded with numerous 

 acicular crystals of hornblende ; while scattered through it are dark 

 segregation-patches mainly composed of the same mineral. 



Microscopic examination of the specimens obtained by Mr. Hobley 

 shows that they agree closely enough to have come from the same 

 dvke. The specimens differ mainly in coarseness of grain. 



The phenocrysts are small but numerous, and consist of plagioclase, 

 hornblende, and augite. The most conspicuous are the small needle- 

 shaped hornblendes, which are idiomorphic ; the species is the 

 brown basaltic hornblende, having extinctions which, measured 

 from c, vary from 0° to 8°, and a pleochroism ranging from dark 

 brown to golden yellow ; many of the crystals are twinned. 



The pyroxene also is idiomorphic ; its colour is light green ; its 

 extinction, measured from e, is over 35° ; the species is, therefore, no 

 doubt augite. The plagioclase mostly occurs as small, long, and lath- 

 shaped crystals, but a few large felspar-phenocrysts occur in the 

 rock, llmenite is fairly common, and apparently has used up all the 

 titanium, for the pyroxene does not show the pleochroic border of 



1 Map of the Anglo-German Boundary Survey in Equitorial East Africa, 

 sheets 3 & 4, I. D. ; W. O., Kos. 1003 & 1004. 



