Yol. 6^.~] PETROGRAPHY OF THE ZAMBEZI BASIX. 211 



but these are cemented by doleritic material which is of the nature 

 of a flow and not fragmental. 



Zeolites. 



The zeolites collected separately include stilbite, mesolite, and 

 laumontite (?). Stilbite and laumontite (?) are associated in the same 

 mass, the stilbite being pinkish and the supposed laumontite opaque - 

 white. The mesolite occurs in colourless, translucent, radiating 

 masses. 



The Sediments, etc. 



Boomka Flags. 

 F 1038. Locality: Sandstone, 6 miles north of Geise's, Deka valley. 



A fine-grained quartzose sediment, iron-stained, and made up of 

 very angular grains measuring not more than 0-15 millimetre in 

 greatest dimension ; certainly not wind-blown. 



Surf ace -Deposits. 

 F 1044. Locality: Limestone, 10 miles north of Deka valley at Geise's. 



This rock is yellowish-grey, compact, and fine-grained, with white 

 patches and streaks. Under the microscope the main mass of the 

 rock is seen to be composed of a calcareous paste which has under- 

 gone little or no silicification, and a few small patches of recrystal- 

 lized calcite. The white patches are seen to consist of well-rounded 

 quartz-grains, with some equally well-rounded limestone-grains set 

 in a fine detrital calcareous cement. The limestone is probably of 

 organic origin, and contains many sections of organic remains which 

 may be referred to Cyprids. 



It is suggested that the streaks and patches are solution- tubes 

 and cavities filled with blown sand. 



F 1045. Locality : East of Mavangu Camp, north of the Zambezi. 



A pink quartzite, made up of millet-seed grains of quartz and 

 microcline, with a little finer material of subangular character. 

 The matrix is chalcedonic. The large proportion of microcline 

 would seem to indicate derivation from an area of crystalline 

 schists and gneisses. Probably this is a desert-sand solidified by 

 secondary silicification. 



F 1046. Locality : Bed of the Gongobujo Eiver, Makwa. 



Greenish-grey siliceous rock, with white calcareous patches. It 

 is similar to the above, with the exception that it is not red-stained 

 and contains a less quantity of microcline. The matrix between 

 the grains is chalcedonic, but in this case evidently replaces an 

 original calcareous cement. In the white patches the replacement 

 of the calcareous material has gone on to a very small extent. The 

 white patches seem, therefore, to represent more or less the original 

 character of the rock. 



Pebbles from the Gwemanzi River, north of the Zambezi, include 

 several schists and gneisses, among which may be mentioned a 

 quartz-biotite-gneiss and a garnetiferous pegmatite- 

 gneiss. 



