Yol. 69.] METAMORPHOSED SEDIMENTS IN BRITISH EAST AFRICA. 537 



are not rare. A few irregular grains of calcite, presenting the 

 appearance of an original mineral, are noteworthy. 



With a view to the study of the arenaceous sediments, if they 

 may be so termed, intercalated between the beds of marble, a thin 

 section has been prepared of a rock which is almost entirely a binary 

 compound of quartz and felspar. Its thickness was some 18 inches. 

 The calc-rocks between which it was found were of different 

 facies: the upper coarse, the lower much finer in grain. The rocks 

 of the section were decomposed, and the section overgrown, but my 

 impressions in the field were that the gneiss did not represent a 

 sill. Most of the felspar is unstriped, but the rock contains a 

 little acid plagioclase and microcline. Some secondary white mica, 

 derived from felspar, and a few original flakes of a reddish-brown 

 biotite, also occur. As appears to be usual in these rocks, the 

 microcline is late in consolidation, and has corroded the other 

 minerals. In this rock also the tendency of the quartz to form 

 elongated vermicular phacoids and extremely irregular grains, with 

 coral-like apophyses, is worthy of note. This structure appears to 

 be characteristic of certain rocks of this series. 



The same peculiarity is more marked in another example, also 

 a biuary compound of quartz and felspar, and found between 

 marble beds in the stream known to the Masai as II Bisil, south 

 of the drainage-basin of the Turoka. The rock consists of an 

 aggregate of quartz and felspar, with a few flakes of biotite and 

 muscovite. The foliation is distinct, the quartz forms long grains 

 which usually are transversely cracked, and suggest that the rock 

 was originally a coarse grit- or pebble-bed. In some instances, the 

 quartz-grains have been clearly formed before the microcline, the 

 crystals of which abut against its surface. An acid plagioclase is 

 an important constituent. The biotite has a marked absorption : 

 C is brownish red, as in some kinzigites, and A. is of a pale straw- 

 colour. 



Intercalated between marble beds from the same locality (II Bisil) 

 was collected another rock, having some relation with the kinzi- 

 gites and with the last-described rock. It is composed of pink 

 garnets, lapped about by a red-brown mica, in a base of quartz 

 and water-clear felspar, the former predominating. The angles of 

 extinction of the latter are small, but much of the felspar is un- 

 twinned. The quartz shows the elongated blunt-ended lenticles 

 noticed before. Small granules of zircon also occur, and one or 

 two of a mineral which appears to be the kyanite of other slides. 



Similar rocks, having affinities with the kinzigites, were found in 

 the stream-bed first mentioned, and about 300 yards from the 

 recorded section. 



The rock, a garnet-biotite-gneiss, contains some quantity 

 of magnetite and ilmenite, the former being the commoner. The 

 biotite is of the usual straw-coloured variety customary in these 

 rocks, and quartz preponderates over felspar, the greater part of 

 which is unstriped. There is no microcline. Zircon and apatite 



