534 ME. J. PARKINSON ON A GROUP OF [Dec. I913,. 



24. On a Group of Metamorphosed Sediments situated between 

 Machakos and Lake Magadi in British East Africa. By 

 John Parkinson, M.A., E.G.S. (Read June 11th, 1913.) 



[Plate LI — Microscope-Sections.] 



In the latter half of 1911, I had the opportunity of examining a 

 group of crystalline rocks situated immediately below the southern 

 edge of the great lava-plateau of the Kapiti Plains, and embracing 

 many varieties of reconstructed sediments, from quartz-schists to 

 highly crystalline marbles. Scattered outcrops of similar rocks 

 have been described by Mr. Maufe from other localities in the 

 Protectorate. 1 The rocks here described were found in approxi- 

 mately lat. 2° S. and long. 37° E., and, so far as I was able to 

 follow them, form the ground drained by the headwaters of the 

 Turoka River, a designation which I think might well be applied 

 to the group, as the name is one of the few found on the usual maps 

 of the district, and is likely in the future to become more widely 

 known as an important station on the Magadi Branch Railway. 

 The section to which I desire to draw attention is found on the 

 right bank of a stream immediately south of the old ' safari ' route 

 to the Lake, about a couple of miles above the site of Turoka 

 Station. 



In apparent ascending order, the following rocks are met 

 with : — 



(1) At the base, a hornblende-schist, of which about 

 3 feet 5 inches is visible. A thin section of this rock shows large 

 irregular plates of the common green hornblende, contained in a 

 rather granular mosaic of water-clear plagioclase and quartz. The 

 felspar is often untwinned, and commonly exhibits graduated 

 zoning. In one part of the slide, diopside takes the place of the 

 hornblende ; sphene, apatite, zircon, and rarely a colourless idio- 

 morphic epidote, are accessory minerals in a rock which calls for no 

 special comment. 



(2) This hornblende-schist is overlain by a flaggy and impure 

 marble, 3 feet thick, and this in turn by 



(3) A fine-grained biotite-gneiss, 2 feet thick. 



This rock is of the ' pepper-and-salt ' type, that is, a non- 

 porphyritic rock, in which the black and white minerals are of 

 small dimensions and approximately equal proportions. The 

 biotite-flakes are about 1 mm. long, and constitute roughly a third 

 to a half of the -whole. 



Unfortunately, this and the succeeding members of the section 

 have been rendered so friable through weathering, that I found it 

 impossible to procure a fragment from which a section could be- 



1 See Colon. Eep. Miscell. No. Od 3828, 1908, p. 29. 



