NATURAL HISTORY OF AFRICA. 42? 
though the superior rock, is of older formation 
than the granite. The granite, therefore, is a mi-* 
neral that has come up from below into the situa- 
tion it now occupies, and is not one of which the 
materials have been deposited by the sea in any 
shape, either mechanical or chemical.* If this ex- 
pfanation was admitted, then we would be forced to 
maintain the igneous origin of sandstone, lime- 
stone, clay slate, and many other rocks, admitted in 
all systems to be of Neptunian origin, because these 
rocks exhibit similar phenomena to those just men- 
tioned, as occurring at the junction of the granite 
and slate. 
Gneiss. 
This rock, which is a compound of felspar, quartz, 
and mica, with a slaty structure, is met with in Up- 
per Egypt, and probably in some of the other gra- 
nite districts already mentioned. 
Mica Slate. 
I do not find this rock mentioned by any of the 
African travellers. 
Clay Slate. 
This well known mountain rock occurs abundant- 
ly in some districts in Upper Egypt, where it is tra- 
versed by thick veins of jasper, and also at the 
* Hall and Playfair in Edinburgh Philosophical Trans* 
actions, Vol. VII. p. 277. 
