GEOLOGY OF NI6RITIA. 265 



frequency of oases, even their total destruction by blowing 

 sand as we advance westward.* 



What is the Geognostical Age of the Sahara?' — Many are 

 of opinion that the Sahara must at one time have been the 

 bed of the ocean. The very frequent saUne impregnation 

 of the sand, the rolled pebble and sands mixed with sea- 

 shells at the foot of the southern acclivity of the Atlas and 

 other parts of the Desert, are considered as in favour of this 

 hypothesis. At what period did this great tract rise above 

 the waves of the ocean 1 This can only be guessed at by 

 an attentive examination of the junctions of the sandstones, 

 limestones, &c. with the bounding primary ranges of the De- 

 sert. If they are the same on the south side as on the north 

 or Atlas side, then it would follow that the Desert rose above 

 the sea at the time when the Atlas made its appearance 

 from below; that is, after the deposition of the tertiary 

 rocks, — at a period when the earth and its animals and 

 vegetables were nearly the same as at present. 



3. Geology of the Region to the South of the Sahara, and 

 to the North of the Great Table-land. — This is the Land of 

 the Negroes, called also Soudan or Nigritia. The high 

 land on the west of tliis part of Africa is partly accumu- 

 lated around the sources of the rivers Senegal, Gambia, 

 Rio Grande, and Niger or Joliba. From the sources of the 

 Niger the mountains run eastwards, under the name Kong 

 Mountains, across Africa, when at length they are said to 

 form a junction with the Mountains of the Moon, that range 

 onward and join with the vast alpine land of Abyssinia. 

 Parts of this boundary are very lofty, some mountains of the 

 Kong chain attaining an elevation of 14,000 feet above the 

 level of the sea. From the meager details of travellers m 

 regard to this part of Africa, all we can infer is, that the 

 mountains on the west and along the south of this zone 

 contain primitive rocks of various descriptions, as granite, 

 mica-slate, clay-slate, quartz rock, hornblende rock, lime- 

 stone, &c. In different parts these rocks seem traversed 

 by augite greenstone or secondary traps. The secondary 

 sandstones and limestones connected with these ranges not 

 having been accurately described, we cannot venture any 



* The long continuance of the easterly in comparison of the westerly 

 vriad, may explain how it happens that the whole country of Egypt baa 

 not ere this *•«?«» swallowed up by the saud-flood of the Desert. 

 Z 



