LOCALITY 49 



jr large rivers such as the Nile, the Congo, or the 

 Amazon, the atmosphere takes up from them a large 

 amount of water by evaporation, this amount varying 

 from a daily mean, for the year, from about 3 to 10 

 millimetres, according to whether the locality is in a 

 heavy rainfall zone or not. This saturation of the 

 itmosphere, where it can be effected, has a distinct 

 influence on the general character of the forest, which 

 may have the appearance of a rain-forest, with a large 

 preponderance of evergreen trees, even where the rainfall 

 is considerably below 1900 millimetres (75 inches). My 

 own experience ranges only from the coast forests of 

 Ceylon, which are mostly evergreen, with a rainfall of 

 down to 875 mm. (35 inches), to the evergreen scrub- 

 forests of Erkoweit in the range of hills fringing the 

 west coast of the Red Sea. The former I have already 

 described. In many places they come right down to 

 the sea-beach, where the trees are twisted and stunted 

 by the w T inds, and while in the northern part of the 

 island, where there are no mountain ranges to speak 

 of, they extend from the east to the west coast with a 

 small interruption made by thorn-forest caused by a 

 pan of heavy black clay or " cotton-soil," farther south, 

 on the drier east coast, the evergreen belt stretches up 

 to about 50 miles from the coast. 



The Erkoweit plateau, situated at about 1000 metres 

 (above 3000 feet) above the sea-port of Suakin, is 

 another instance of how a variety of factors may combine 

 and constitute a certain class of forest. The majority 

 of the neighbouring hills are composed of the red Nubian 

 sandstone, which absorbs both heat and moisture without 

 retaining the moisture. The moisture-laden winds from 

 the sea are therefore insufficient to clothe these slopes 

 with more than sub-desert flora, such as fleshy and 

 thorny plants not unlike those found in Mexico, viz. : 

 thorny Euphorbias, occasional stunted Acacias (A. 

 etbaica), fleshy Zygophyllaceae, Ficoideae, Asclepiadeae, 

 Crassulaceae, Aloes, and Tree-lilies [Dracaena), and 

 thorny, stunted Acanthaceae. The outcrop of granite 



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