THE GEOLOGY OF THE SAHARA. 379 



crystalline mass, which occupies most of the Nubian desert 

 and sends a long and narrow tongue along the shores of 

 the Red Sea as far as the Gulf of Suez. This great 

 Archaean mass forms the bed of the Nile for a con- 

 siderable distance, but it nowhere extends far to the west 

 of that river. 



Palceozoic. — The gneiss of the Kong Mountains is 

 succeeded to the north, in Senegambia, by the extensive 

 area of slates and quartzites which Lenz refers with doubt 

 to the Silurian. He found the same beds at intervals 

 between the Igidi Desert and Timbuktu ; and it would 

 appear therefore that these rocks form a belt stretching 

 from Senegambia towards the north-east (22). 



They are succeeded on the north-west by a second 

 broad belt of later beds, which Lenz found to extend 

 from the Wadi Draa to the granites of El Eglab. These 

 consist of limestones and sandstones ; and at Jerf-el-bir 

 and other places south of Tenduf, he obtained crinoids, 

 brachiopods, and other fossils in abundance. His collections 

 have been examined by Stache, and include numerous species 

 of Productiis. Spinfer, Chonetes, Streptorhynchus, and other 

 brachiopods also occur ; Cyathophyllum and other corals ; 

 crinoids, including Poteriocrinus ; and a variety of polyzoa 

 and other forms (31). The general character of the fossils 

 is undoubtedly Carboniferous, but a few of the specimens 

 show Devonian affinities. 



The northern boundary of the Palaeozoic rocks is drawn 

 by Lenz along the foot of the Atlas as far as the desert 

 El Erg. On the other side of this the Cretaceous beds of 

 the Mzab, El Goleah, and In Salah form a plateau with an 

 escarpment running from north to south. The boundary 

 of the Palaeozoic rocks must therefore turn to the south 

 under the sands of the Erg, and it probably lies along the 

 foot of the plateau. 



At In Salah the escarpment turns towards the east. 

 The plateau is formed of Cretaceous beds ; but the rocks 

 on which they rest are Palaeozoic, — for Ismail-Bou-Derba 

 found Devonian beds at the foot of the plateau near 

 Temassanin (30, p. 63). East of Temassanin the boundary 



