PRIMITIVE ROCKS* 
433 
We do not possess any extensive series of ob- 
servations in regard to the general direction and 
dip of the African strata, and the relative posi- 
tions of the great rock formations have not been 
much investigated. We can discern, however, in 
the accounts of travellers, descriptions of species 
of three of the great classes of rocks, viz. Pri- 
mitive, Secondary, and Alluvial ; and some ac- 
counts would appear to intimate the occurrence 
of volcanic rocks in this quarter of the globe. 
Of these rocks the alluvial occupy an uncom- 
monly large portion of the surface of the conti- 
nent, while the volcanic are but of comparatively 
small extent. The primitive rocks occupy the 
higher and middle parts of several of the great 
tracts of mountainous country, but the secon- 
dary rocks which rest upon these, generally 
occur at a lower level. Petrifactions, or fossil 
organic remains, are met with in great abundance 
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. 
The primitive rocks mentioned by travellers as 
occurring in Africa are granite, gneiss, mica 
slate, clay slate, quartz rock or sandstone, por- 
VOL. II, 23 e 
