352 H. B. Neicton — Fossils from Nuhian Sandstone, Egypt. 



MetamorpMc : Hornblende-schist (Bulawayo). 

 Chlorite -schist (Gatooma). 

 Schistose Serpentine (Sebakwe). 

 'Banded Ironstone' (Hopefountain). 

 As regards the sediments cited in the two first instances, the laterite 

 may perhaps be regarded as of detrital origin, at least in part. In all 

 the other instances it is due to the decomposition iti situ of the lander- 

 lying rocks, whose nature is reflected in the constitution of the 

 resulting laterite. 



Composition. — The commercial importance attaching to the highly 

 aluminous varieties of laterite has led to their being given much 

 prominence in discussing its origin, and perhaps rather obscured the 

 fact that such deposits are of limited extent. The ordinary forms of 

 laterite are usually more or less conglomeratic, owing to the inclusion 

 of abundant fragments of vein-quartz and of the underlying rocks, as 

 well as drifted sand, etc. The following determinations of the more 

 important constituents of three Rhodesian samples of laterite will 

 serve to illustrate this point. 



A. B. C. 



Iron oxide, Fe-j O3 . . 14-71 32-14 34-8 



Alumina, AI2 O3 . . . 1-41 -22 25-9 



Combined water, H, . 4-44 6-37 not det. 



Insoluble ( + silica) " . . 75-10 60-69 29-2 



A represents a light-coloured laterite resting on granite, from the 

 Matopos ; B a dark-coloured sandy laterite resting on hornblende- 

 schist, near Bulawayo. Both of these were average bulk samples 

 collected by the writer, in which the iron and alumina were first 

 separately determined by the caustic alkali method and then checked 

 by titration of the iron. C represents a reddish laterite from Northern 

 Rhodesia presented to the Rhodesia Museum by Mr. L. Ludlow. 



A and £ probably represent the average composition of normal 

 laterite much more nearly than most published analyses. As regards 

 more exceptional varieties, very little investigation would probably 

 reveal much more aluminous types than C. Some of the laterites 

 from the Yictoria Falls region are known to be very rich in iron, 

 almost pure limonite, in fact. Although the underlying rock is 

 basalt, they are very free, as a rule, from alumina. Some of the 

 Somabula laterite contains a great deal of manganese, and must 

 approximate in composition to wad. 



Y. — On some Fossils fkom the Nubian' Sandstone Seeies of Egypt.^ 



By E. BuLLEN Newton, F.G.S. 



(WITH A MAP, PLATE XIX.2) 



Intkoduction. 



DR. "W. F. HUME, Superintendent of the Geological Survey of 

 Egypt, has recently sent to the Geological Department of the 

 British Museum, for determination and report, a few Pelecypod shells 



1 Pubhshed by permission of the Trustees of the British Museum. 



2 Plates XX and XXI, giving figures of the fossils, will appear with the concluding 

 part of the author' s paper in September. 



