48 Capt. Campbell on the Geognosy of the Island of Ascension. 
The whole island has a most forbidding and rugged aspect. 
Its highest mountain, named Green Mountain Peak, is 2818 
feet above the level of the sea. The largest portion of the moun- 
tain is 2000 feet above the sea ; and at this height there is a space 
of comparatively level ground, in which the principal garden in 
the island is situated. From the top of the Peak down to about 
this level, or a little lower, the surface, excepting where it is pre- 
cipitous, is covered with a coat of soil, which is nowhere deep, 
and having under it masses of pumice and lava. The precipices 
around this height, are, in many instances, formed of slaggy lava ; 
and, in the lava, are veins filled with opal, containing imbedded 
fragments of vesicular and slaggy lava. In other parts, there are 
rocks of a felspar or trachyte porphyry. Among the many ridges 
shooting from the Green Mountain (M of the chart), one of the 
most remarkable is that composed of black and dark-green perfect- 
ly formed obsidian, which, in some places, is disposed in balls and 
globular concretions, like that found in Kamtschatka ; and, in 
others, in large globular concretions, like those of basalt and green- 
stone. Associated with it there are grey varieties of pearl-stone *. 
This vitreous mineral is there associated with various porphyries, 
apparently trachytic ; and, in some places, green pitchstone, with 
imbedded sphserulite and common pumice and pumice-conglome- 
rate, occur. Not far from the obsidian ridge, there is a remark- 
able hill, named by the sailors The Devils Riding-School , 
marked in the chart P. It is about 700 feet above the level of 
the sea, and between 400 and 500 feet above the surface of the 
surrounding base. It has a circular hollow on the top, which 
probably was formerly much deeper than at present, it being 
now filled up to within 30 feet of the edge of the crater. This 
hill, as far as can be made out from the specimens brought 
home, appears to be composed of trachytic rocks. In some va- 
rieties, the basis is like claystone, and contains imbedded por- 
tions of slaggy lava ; in others, the basis is of felspar, with im- 
bedded crystals of glassy felspar, and fragments of slaggy lava ; 
and the trachyte porphyry sometimes contains, in its cavities, 
* Specimens of vesicular iron-ore were found in a trachyte ridge not far from 
the obsidian ; and also crystals of specular iron-ore, like that of the Island of Strom- 
boli. 
