154 dr. f. Oswald on the [June 1 9 14, 



compact rock may be termed a zoisite-hom stone 1 : it is probably 

 an amphibolite that has been altered by the intrusion of a neigh- 

 bouring diabasic rock, also greatly altered and deeply weathered, but 

 possibly a teschenite 2 originally. The last-named rock crops 

 out of the weathered debris at less than a mile from the outcrop of 

 the altered rock, and occupies the Rungwe Valley at Langueh, a 

 little Indian trading-station, 18 miles from Kisii Boma. 



From the Kona Plateau there had been a slow but steady 

 descent, although the view to the eastward was blocked by low 

 hills and ridges, probably of amphibolites ; but beyond Langueh 

 (4651 feet) these hills flattened out, and on the north and north- 

 west lay a nearly treeless and extensive volcanic plateau, traversed 

 only by shallow valleys extending north-eastwards towards the 

 Kavirondo Gulf. Between Langueh and the Ogweyo River, the 

 low south-westerly and north-easterly ridges are composed of a 

 dark-green tuff 3 derived from a rather acid andesitic lava; I 

 did not, however, find this lava in situ. 



The summit of this lower plateau is composed of the nepheline- 

 basalt already described (p. 141), which probably flowed out from 

 Ruri or Grernbe as a centre, and the remainder of the country as far 

 as Homa Bay is occupied by this basalt and its tuffs (see p. 143). 



1 The rock has a pitchy lustre and splintery fracture, and presents a some- 

 what brecciated appearance. Larger clearer patch.es, usually with prismatic 

 and rectangular outlines, are composed of a mosaic of quartz -granules, 

 flakelets of white mica and interstitial chlorite, and may be pseudomorphous 

 after felspar. Other aggregates consisting of zoisite and quartz show outlines 

 recalling hornblende. Zoisite is abundant, and is honeycombed with quartz, 

 presenting a micropoecilitic appearance ; the largest of the zoisite-crystals are 

 prismatic, in a radiating sheaf. Small specks of kaolin are frequent, and 

 nests of epidote and calcite occur scattered in the rock, which is traversed 

 by thin veins of quartz and epidote. 



2 The rock contains abundant crystals (measuring up to 4 mm. in longest 

 diameter) of plagioclase (acid labradorite, Ab 1 An 1 ), mostly of pris- 

 matic habit, very turbid and decomposed, largely replaced by flakelets of 

 white mica, and traversed by numerous cracks filled with chlorite and occa- 

 sionally epidote ; there is an instance of a granophyric intergrowth of turbid 

 felspar and quartz. Colourless augite (malacolite) occupies the bulk of 

 the slide, having crystallized later than the felspar, in aggregates of allo- 

 triomorphic crystals (measuring up to 3 mm. in longest diameter) with a 

 tendency to form long prisms ; it contains small diallagic rods ; it is sometimes 

 twinned and intergrown in a pegmatitic manner, so that a fresh crystal may 

 be intergrown with a uralitized crystal. Zeolitic pseudomorphs, possibly after 

 elaeolite, are pale brown and minutely fibrous, polarizing like moire silk in 

 low greys and extinguishing parallel to the fibres ; they enclose small patches 

 of augite in a pegmatitic intergrowth, especially centrally. Some clear 

 secondary green hornblende is present, developing at the expense of the 

 augite, and is associated with some quartz. Apatite and magnetite are 

 accessory. 



3 The rock is compact and very fine-grained : numerous fragments of 

 crystals are present, but all of very small dimensions : — Abundant plagioclase 

 (oligoclase-andesine, Ab n An 2 ) ; alittle orthoclase; many angular frag- 

 ments of very pale-green di op side, quite fresh and free from inclusions, 

 sometimes exhibiting lamellar twinning ; numerous microlitic felts of oligo- 

 clase, colourless augite-granules and interstitial glass ; and lapilli of brown 

 dusty glass with zeolites filling the steam-cavities. Chlorite is abundant 

 interstitially, and a subordinate amount of zeolite and calcite is present. 



