152 GEOLOOr. 



The plateau contracts greatly towards the north. The 

 last of the great aflSuents of the Nile in this direction is 

 the Mareb, which rises in Hamazen, almost due west of 

 Massowa. The streams running from the northern side 

 of Hamazen, the principal of which are the Anseba and 

 the Barka, unite and flow into the Eed Sea south of 

 Suakim. Although a few isolated plateaux of 7,000 to 

 8,000 feet, such as those of Mensa and Marea, exist in 

 the area drained by these rivers, the elevated country 

 is of smaD extent, and the greater portion of the 

 surface does not rise above 4,000 to 5,000 feet above 

 the sea. 



The fundamental rocks of all Tigre, and probably of 

 all Abyssinia, are metamorphic. They compose the mass 

 of the table-land, and while they occupy no inconsider- 

 able proportion of its surface, they a.re exposed, in Tigr^ 

 at least, in every deep valley. 



As a general rule, it may be asserted that, in the neigh- 

 bourhood of the route followed by the British army, so 

 much of the country as exceeds a height of about 8,000 

 feet above the sea consists of bedded traps. There are 

 a few exceptions, such as the sandstone range of Sowera, 

 near Senafd, but they are neither numerous nor impor- 

 tant. The traps of Ashangi also descend to a much 

 lower level than 8,000 feet above the sea. The greater 

 portion of Central and Southern Abyssinia, as far south as 

 Shoa at least, is known to consist of the same rock.^ 



Between the traps and the metamorphics a series of 

 sandstones and limestones intervenes, one group of the 



^ It is shown by M. Rocher d'H^ricourt to be the case in Shoa. 



