Tol. 70.] MIOCENE OF THE TICTOEIA XYA.XZ.V, ETC. 153 



brown sandy soil, and the wide valley of the Nyakuru which 

 traverses this rock only shows gentle rounded slopes of 8°. 



The summit of the plateau at the 13th mile from Kisii consists 

 of a pink granitic biotite-gneiss 1 ; but the porphyroid soon re- 

 appears, weathering to a depth of over 6 feet of yellow sand. 

 Apparently overlying it occurs a small outcrop of a greenish- 

 orev chloritic calc-schist. 2 



On descending from the Kona Plateau, one notes that the rock 

 at the 14th mile from Kisii (before coming to Sori Kodongo), 

 becomes highly schistose and mylonitic, consisting practically of 

 very fine mosaics of quartz with sericitic mica and much limonite, 

 readily decomposing to a reddish-brown sandy soil. It is, perhaps, 

 the greatly- crushed marginal modification of the porphyroid, 

 forming a parallel to the sericitic schists of the Kuja Gorge 

 below Yinvo. As I proceeded, I found the underlying rock to be 

 extremely ferruginous, weathering so deeply that no fresh rock could 

 be reached. Much murrain has formed in patches, as it always 

 does where there is much iron in the soil. Xumerous veins of 

 quartz, a foot thick, run in a north-westerly and south-easterly 

 direction : that is, in the same direction as the strike of the 

 gneisses and schists, and in evident relation to the earth-movements 

 that have affected this district. The low rounded ridges in this 

 area belong to the amphibolite- group, but the rocks are altered 

 and decomposed to an excessive degree. 



Shortly before one gets to Langueh, the rock is seen to be a 

 greenish-grey fine-grained epi dote- schist, 3 probably a much- 

 altered amphibolite, and half a mile farther on the very dark-green 



1 The rock contains numerous allotriomorphs (measuring up to 7 mm. 

 in longest diameter) of orthoclase (microperthite) with microperthitic 

 interpenetration ; it is turbid with dusty decomposition-products, flakelets 

 of white mica, etc., and hamaatitic dust. Plagioclase (oligoclase-andesine, 

 Ab2An!), also somewhat decomposed, is subordinate. Quartz is abundant 

 in separate grains (up to 3 mm.), as well as in granophyric intergrowths 

 with the felspars ; it is much cracked, and sometimes brecciated marginally ; 

 it contains some biotite and also calcite. Bio tit e (up to 2 mm.) is some- 

 what sparse ; it encloses magnetite, and has been entirely altered to chlorite 

 (partly spherulitic and exhibiting the usual blotchy pleochroism), with epidote, 

 calcite. and some haematite. Sphene bordered with haematite is accessory, 

 and calcite occurs along cracks. 



- The rock is mainly composed of aragonite and spherulitic chalce- 

 dony in alternating zones, with some chlorite and much sericitic mica in 

 interstitial aggregates. Numerous small curved markings are present, circular, 

 ellipsoidal, or kidney-shaped, and in some cases somewhat compressed and 

 drawn out, all approximately of the same size ; they are evidently anterior to 

 the formation of the chalcedony or the white mica, which traverses them 

 independently. They are finely granular on the concave side, bluish black by 

 transmitted and white by reflected light. They somewhat recall the tests of 

 Cypris. but Prof. Bonney, who has kindly examined the slide, considers them 

 to be merely extraneous matter thrust outwards during the formation of the 

 spherulitic chalcedony. 



3 This rock consists of schistose aggregates of epidote and quartz - 

 granules, pistacite and zoisite, and some yellow-green serpentinous 

 mineral showing aggregate polarization. Much limonite is present in strings 

 and patches. The rock has been considerably fractured and veined with quartz. 



M2 



