200 THE BOTANY OF ASCENSION. [Sess. Lxx. 
The island comprises an area of some forty square miles of 
undulating plains lying around the base of Green Mountain, 
a tertiary voleano which rises to a height of 2840 feet. 
The geological constitution of the island is hard volcanic 
slag and some beds of volcanic ashes.1 With the extreme 
dryness of the atmosphere, the want of rain, and the equa- 
bility of temperature at sea-level, the low-lying ground 
remains almost as fresh and unweathered to-day as if its 
formation was a matter of only a few years ago instead of 
ages. The vegetation of these dry and soil-less plains is 
naturally very scanty; in fact, save in some exceptionally 
favoured spots, they are practically a desert: but that want 
of water is the one vital hindrance to vegetation is more 
clearly seen as one ascends Green Mountain. The geological 
structure and soil of this old volcano is of course essentially 
the same as that of the plains, but the vegetation steadily 
increases from the foot upwards, until before 2000 feet one 
is pleasantly surprised to find oneself amidst a veritable oasis 
of rich sub-tropical vegetation. Still higher the vegetation 
assumes a more temperate aspect, and the top, exposed to 
the continually blowing south-east trade wind, is covered with 
grassland. This position of the island, in the direct track of 
the dry south-east trade winds, is responsible for the ex- 
tremely small rainfall, which at Georgetown (sea-level) 
averages under three inches a year; but on Green Mountain, 
at a height of 2000 feet, where clouds often envelop the 
hill, it is over 17 inches. With this very scanty rainfall 
the extreme desert nature of the plains is little to be 
wondered at, and it was only at “ Wideawake Camp,” the 
nesting-place of myriads of terns (Sterna fuliginosa), that 
much vegetation was found, and it was almost entirely 
composed of Portulaca oleracea, considered to be indigenous, 
and the widely spread tropical grass Setaria verticillata, a 
species doubtless introduced by the action of the terns. 
“Wideawake ” is a hollow apparently slightly less arid 
than its surroundings, while, in addition, the guano of so 
many birds must materially assist the vegetation. Portulaca 
oleracea in places forms an almost continuous carpet, and 
is apparently well adapted to the prevailing conditions, for 
' For a fuller account of the geology of Ascension, see Darwin’s 
“ Naturalist’s Voyage.” 
