436 NATURAL HISTORY OF AFRICA. 
time, and in the same manner. It is remarked 
by Professor Playfair, that the penetration of the 
slate by veins, from the mass of granite which it 
surrounds, proves that the slate, though the supe- 
rior rock, is of older formation than the granite. 
The granite, therefore, is a mineral that has come 
up from below into the situation it now occupies, 
and is not one of which the materials have been 
deposited by the sea in any shape, either mecha- 
nical or chemical.* If this explanation be ad- 
mitted, we must maintain the igneous origin of 
sandstone, limestone, clay slate, and many other 
rocks, admitted in all systems to be of Neptunian 
origin, because these rocks exhibit similar pheno- 
mena to those just mentioned, as occurring at the 
junction of the granite and slate.t 
2. Gneiss. 
This rock, which is a compound of felspar, 
quartz, and mica, with a slaty structure, is met 
with in Upper Egypt, and probably in some of 
the other granite districts already mentioned. 
* Hall and Playfair in Edinburgh Philosophical Transac- 
tions, Vol. VII. p. 277. 
t The mineralogy of the Cape of Good Hope has lately 
engaged the attention of my friends Dr Adams and Captain 
Carmichael, and of my former pupils Captain Wauchope 
and Clark Abel, Esq. Mr Abel in his wofk just published, 
communicates several curious facts in regard to the minera- 
logy of this district. 
