NATURAL HISTORY OF AFRICA. 425 
been made in regard to the general direction and 
dip of the African strata, and the relative positions 
of the great rock formations still remain very im- 
perfectly known. We can discern in the accounts 
of travellers descriptions of species of four of the 
great classes of rocks, viz. Primitive, Transition, 
Hcetz, and Alluvial ; and some accounts would ap- 
pear to intimate the occurrence of volcanic rocks 
in this quarter of the globe. Of these rocks the 
alluvial occupy an uncommonly large portion of 
the surface of the continent, while the volcanic are 
but of comparatively small extent. The primitive 
and transition rocks occupy the higher and middle 
parts of several of the great tracts of mountainous 
country, but the floetz rocks which rest upon these 
generally occupy a lower level. Petrifactions, or 
fossil organic remains, are met with in great abun- 
dance in some of the newer rocks ; and of these the 
petrifactions of vegetables are generally imbedded 
in slate or sandstone, while those of animals are 
contained in limestone. 
Primitive Rocks. 
1. Granite. This rock, which is a compound of 
felspar, quartz, and mica, is met with in Upper 
Egypt, as near to Cosseir ; also at Tetuan in Al- 
giers, in Darfur, and in the great ranges of moun- 
tains at the sources of the Senegal. The Kham- 
kiesberg, to the northward of the Cape of Good 
