440 Miss C. A. Raisin—Geology of Africa. 
deposition from water, but below the level of High Nile, whence 
most of the specimens were obtained, we note in photographs taken 
by Mr. Scott Elliot that sand is piled up abundantly around the 
hard rocks. Also the streaked and furrowed surface resembles sand- 
polished rocks from elsewhere. 
From these specimens we can make only a few general inferences 
—and those not entirely new—as to the order of succession. It 
seems probable that granite and gneiss near the first cataract, and 
diorite near the second, form a fundamental crystalline series. At 
some later epoch or epochs, igneous irruptions occurred of acid, 
intermediate, and basic rocks. Among these are the numerous 
mica-traps, or mica-diorites from near Assouan. Near Wady Halfa 
felstone and granite seem to cut the diorite, while an andesite is 
perhaps still later cutting the granite. Basic magmas subsequently 
were intruded as dykes, or possibly were poured out as lavas. ‘The 
disturbed period marked by igneous action was followed by 
denudation of the lower crystalline rocks and the tranquil deposition 
of the overlying sandstones and arkose. 
The specimens which have been examined may be arranged 
roughly in the following classes, the order of succession in certain 
cases being somewhat hypothetical :— 
1. Crystalline rocks, probably fundamental; granite, gneiss, diorite. 
2. Igneous rocks, probably intrusive. 
(a) Near the first cataract: microgranite, basalt, porphyrite, augite-syenite (°?), 
mica-trap, diabase. 
(6) Near the second cataract: 
(1) Intrusive in diorite, granite or syenite: microgranite, felstone. 
(2) Intrusive in granite: andesite. 
(8) Basalt or dolerite (lava flow or intrusive). 
3. Sandstones and arkose. 
I].—Sprcimens rrom West Arrica (Stmrra Leone). 
Tn a second African expedition, starting from the west coast, Mr. 
Scott Elliot collected about a hundred rock specimens, which also 
have been examined at University College under the supervision of 
Professor Bonney. A detailed account has been furnished to Mr. 
Scott Elliot, and is published in the Colonial Report on Sierra 
Leone,’ but a short summary of the results may be of interest. 
The foliated rocks, of which many are certainly igneous, doubtless 
belong to an ancient series. They include gneiss, and foliated 
granite, sometimes containing hornblende or biotite. But few 
specimens appear to have been severely crushed. The igneous 
rocks, other than granite, are doubtless, in some cases, dykes or 
sills and intrusive masses, but many examples equally well might 
be parts of lava flows, a question to he decided only by field 
examination. Petrologically they are mainly dolerites, often in- 
teresting from the presence of olivine, enstatite or hypersthene. 
A tendency to a spherulitic structure is shewn in one specimen, 
1 Colonial Reports. Miscellaneous, No. 3, Sierra Leone. Reports on Botany and 
Geology, 1893. I regret that, as I unfortunately missed seeing the proof of this 
paper, some printers’ errors in it have escaped correction. 
nee ee eee aimee 
