434 NATURAL HISTORY OF AFRICA. 
are found near the clay slate of Gellbock, in South- 
ern Africa. 
In some maps of Africa, I observe a range of 
mountains to the southward of the syenite and 
porphyry formations of Upper Egypt, marked as 
composed of basalt. 
Volcanic Rocks. 
We have no description of the volcanic rocks of 
Africa ; but travellers inform us, that there is a 
volcano in Abyssinia, and two on the coast of Mo- 
sambique. 
Alluvial. 
The principal alluvial formation in Africa is the 
sand which covers the great deserts. 
In some places of the great desert, as Mr Horne- 
man remarks, the sandy waste was covered with in- 
numerable fragments of petrified wood, sometimes 
whole trunks of trees, twelve feet in circumference, 
sometimes merely branches and twigs, or even 
pieces of bark. This petrified wood was generally 
black, but sometimes ash grey, when it resembles 
natural wood so completely as to be sometimes 
brought in for the purpose of firing. 
It is to be regretted, that we possess no accurate 
description of this sand, or of the minerals con- 
tained in it. Now, since it appears probable, that 
some kinds of sand are original deposites, and not 
