﻿656 ME. J. A. HOWE PETROLOGICAL NOTES [Aug. I905, 



No. 51. — This specimen, said to be from the same locality 

 as the preceding, is a compact rock of slightly-redder colour ; in 

 the hand-specimen it shows the spherical oolitic grains weathering 

 more readily than the matrix, but in the centre of each grain a 

 little heap of yellow granules stands up above the rest. In the thin 

 section the round grains are observed to be slightly smaller than 

 those in No. 42, and the majority of them are almost perfectly 

 spherical ; there are no cases of composite grains. 



The concentric structure characteristic of No. 42 is absent, and 

 instead there is a markedly-radial arrangement of alternating 

 clearer, crystalline, and obscure, dull rays; the latter are generally 

 conical in section, with their bases on the periphery of the grain. 

 The ratio of clear to dull matter is remarkably constant ; the clear 

 matter is always granular calcite, and in no case has any fibrous 

 radial calcite been observed. (See PI. XLIV, fig. 1.) 



In the centre of each grain are irregular granules or small rhombs 

 of a yellowish carbonate, probably chalybite ; these are not strictly 

 confined to the centre, but to some extent appear sporadically within 

 the mass of the grain as well. 



In portions of the slide many of the spheres have been partly 

 obliterated by the crystallization of the calcite of the groundmass, 

 which is usually clear and granular, although a few small flakes of 

 haematite are occasionally included in it. 



The structure of these spherical grains strongly suggests an 

 organic origin for them ; but there can be no doubt as to the 

 inorganic nature of the other oolitic limestone, of which this appears 

 to form a variety. 



Rocks of the Archaean (?) Series. 



The granitic district including the country between the 

 Lipodongu Falls and Poko, between Bima and Amasi, 

 and the granite-ridge between the Rubi and the Uelle 

 Rivers. 



Granites of a similar type were observed to form the substratum 

 of the whole of this district and, with the exception of the diabase 

 reported from Panda, no other kind of rock was found beneath the 

 alluvium. Only two specimens were brought back : No. 54 from 

 the rapids on the Rubi River, where it cuts the granite-ridge ; and 

 No. 52 from a point some 60 miles farther east, near Poko. 



No. 54 (granite-gneiss) is a dark rock of medium grain ; the 

 colour is due to the red staining of the felspars and the abundant 

 spots of mica. The microscope shows felspar in fairly-large, 

 hypidiomorphic crystals, much decomposed and stained with iron- 

 oxide ; granular epidote and chlorite are common in the 

 felspar-mass, as alteration-products. Both plagioclase and ortho- 

 clase are represented. Quartz occurs in broad streaks and grains, 

 and it nearly all shows undulose extinction. A greenish mica is 

 the remaining important mineral. A little hornblende is now 



