444 NATURAL HISTORY OF AFRICA. 
Hope. Most of the African islands abound in 
secondary trap rocks, associated with lavas. 
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. 
Travellers mention one volcano in Abyssinia, 
and two on the coast of Mosambique. The isl- 
and of Bourbon appears to be principally com- 
posed of secondary trap rocks more or less altered 
by heat, and of true volcanic rocks. The Isle of 
France of volcanic rocks and limestone. The Isle 
of Ascension is said to be entirely volcanic, and to 
abound in obsidian. St Helena appears to be a 
mass of secondary trap with considerable beds of 
limestone, or according to some naturalists, of lava. 
Tristan d'Acunha, like Bourbon, appears to be 
volcanic with secondary trap rocks. The islands 
of Goree and Cape Verd are of basalt. The 
Canary islands are composed of volcanic and trap 
rocks, with subordinate^ beds of limestone and 
inconsiderable strata of the primitive class. The 
Azores or Western Isles are of the same general 
nature, and also the Cape Verd and Madeira isles. 
ALLUVIAL, 
The principal alluvial formations are those oxk 
