alluvUl rocks. 
445 
the shores of lakes and the sides of rivers, and the 
vast expanse of sand which covers the great de- 
serts. 
The sand formation is by far the most impor- 
tant of the loose superficial coverings met with in 
Africa. In some places of the great desert, as 
Mr Hornemtm remarks, the sandy waste was 
covered with innumerable 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 com- 
pletely as to be sometimes brought in for the pur- 
pose of firing. 
It is to be regretted, that we possess no accu- 
rate description of ^his sand, or of the minerals 
contained in it. Now, since it appears probable, 
that some kinds of sand are original deposites, 
and not formed by the action of the atmosphere 
and the water on previously existing rocks, it is 
of consequence to know acccurately the oryctog- 
nostic and geognostic relations of sands, particu- 
larly those that form the great deserts in Africa 
and other quarters of the globe. 
SIMPLE MINERALS FOUND IN AFRICA. 
1. Chrysolite, — This gem, described by the an- 
cients under the name topaz, is said to have been 
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