46 Ascension. paet i. 



One white ribbon-like layer of decomposed, pumiceous 

 breccia, was curiously bent into deep unbroken curves, 

 beneath each of the larger fragments in the superin- 

 cumbent stratum. From the relative position of these 

 beds, I presume that a narrow-mouthed crater, standing 

 nearly in the position of Green Mountain, like a great 

 air-gun, shot forth, before its final extinction, this vast 

 accumulation of loose matter. Subsequently to this 

 event, considerable dislocations have taken place, and 

 an oval circus has been formed by subsidence. This 

 sunken space lies at the north-eastern foot of Green 

 Mountain, and is well represented in the accompanying 

 map. Its longer axis, which is connected with a NE. 

 and SW. line of fissure, is three-fifths of a nautical mile 

 in length ; its sides are nearly perpendicular, except in 

 one spot, and about 400 feet in height ; they consist, in 

 the lower part, of a pale basalt with feldspar, and in the 

 upper part, of the tuff and loose ejected fragments ; 

 the bottom is smooth and level, and under almost any 

 other climate a deep lake would have been formed 

 here. From the thickness of the bed of loose fragments, 

 wirh which the surrounding country is covered, the 

 amount of aeriform- matter necessary for their projection 

 mast have been enormous ; hence we may suppose it 

 probable that after the explosions vast subterranean 

 caverns were left, and that the falling in of the roof of 

 one of these produced the hollow here described. At 

 the Galapagos Archipelago, pits of a similar character, 



siderable area ; it lies conformably in the lower part of the stratified 

 mass of ashes and fragments. This substance is of a reddish -brown 

 colour, with an almost metallic lustre ; it is not magnetic, but be- 

 comes so after having been heated under the blowpipe, by which it 

 is blackened and partly fused. This seam of compact stone, by 

 intercepting the little rain-water which falls on the island, gives rise 

 to a small dripping spring, first discovered by Dampier. It is the 

 only fresh- water on the island, so that the possibility of its being in- 

 habited has entirely depended on the occurrence of this ferruginous 

 laver. 



