Report on the Roek-Specimens. 329 



microscope loug clear Carlsbad twins of anorthoclase, with charac- 

 teristic minute twin striations, and one or two small crystals of 

 segirine-augite, are seen in a trachytic groundmass composed of 

 felspar laths showing flow structure, with interstitial tufts of a dull 

 green segirine-augite, a little magnetite, and shreds of a deep red 

 decomposed mineral, probably altered cossyrite or catophorite. 

 Amongst the phenocrysts olivine in rounded fragments surrounded 

 by ffigirine-augite occurs very sparingly (see PI. LIII., fig. 4). 



These characters are almost precisely similar to those presentc.d 

 by phonolitic rocks from the Canary Islands and from the Eift Valley, 

 near Mount Kenya, in Tropical East Africa.^ 



A chemical analysis of the phonolytic trachyte gave the following 



result : — 



SiO, =64-13 



99-91 



Another dark green and brown specimen is a somewhat similar 

 rock, but approaches more closely to the rocks from Mount Kenya to 

 which Prof. Gregory has given the name " Kenyte." ^ In this rock, 

 large porphyritic anorthoclase-felspars (sometimes in glomeroporphy- 

 ritic groups round grains of magnetite, augite and olivine) and smaller 

 phenocrysts of a pale dull green diopside-like augite, and a few red 

 altered crystals of olivine occur in a very hue-grained groundmass, 

 consisting partly of minute felspar-laths and partly of interlocking 

 felspar grains, with a little augite and magnetite. Magnetite grains 

 occur as inclusions in all the phenocrysts and are especially abundant 

 in the augites (see PI. LIII., fig. 3). 



The third specimen shows an approach to the hornblende-basalts. 

 It is a gray rock, showing one or two fairly large phenocrysts of 

 basaltic hornblende. Under the microscope small dull bluish-green 

 phenocrysts of segirine-augite (with high angles of extinction, up 

 to 40°), and a few partially reabsorbed basaltic hornblendes, precisely 



1 See Mill. Ma^., XIII., p. 89, 1901. 



-^ Quart. Journ. Geol. Soc, LYL, p. 205, 1900. 



