Rift valley, overlying a formation of granite, gneiss and quartz.
The rocks of its lower half are mainly granite and gneiss; its upper half is composed of porphyritic greenstone, and a variety of minerals occur.
The Cordillera, which bounds them on the west, is formed of folded beds, while the Sierras which rise in their midst, consist mainly of gneiss, granite and schist.
Bismuth occurs in metalliferous veins traversing gneiss or clay-slate, and is usually associated with ores of silver and cobalt.
Archean rocks - gneiss, schist and granite - cover large areas through which the Nile cuts its way in alternate narrow gorges and open reaches.