chap. in. Aeriform Explosions. 45 



sent state, on a great sandy plain between the rivers 

 Darling and Murray, in Australia, and at the distance 

 of several hundred miles from any known volcanic 

 region. It seems to have been embedded in some 

 reddish tufaceous matter ; and may have been trans- 

 ported either by the aborigines or by natural means. 

 The external saucer consists of compact obsidian, of a 

 bottle-green colour, and is filled with finely-cellular 

 black lava, much less transparent and glassy than the 

 obsidian. The external surface is marked with four 

 or five not quite perfect ridges, which are represented 

 rather too distinctly in the woodcut. Here then we 

 have the external structure described by M. Beudant, 

 and the internal cellular condition of the bombs from 

 Ascension. The lip of the saucer is slightly concave, 

 exactly like the margin of a soup-plate, and its inner 

 edge overlaps a little the central cellular lava. This 

 structure is so symmetrical round the entire circum- 

 ference, that one is forced to suppose that the bomb 

 burst during its rotatory course, before being quite 

 solidified, and that the lip and edges were thus slightly 

 modified and turned inwards. It may be remarked that 

 the superficial ridges are in planes, at right angles to an 

 axis, transverse to the longer axis of the flattened oval : 

 to explain this circumstance, we may suppose that when 

 the bomb burst, the axis of rotation changed. 



Aeriform explosions. — The flanks of Green Moun- 

 tain and the surrounding country are covered by a 

 great mass, some hundred feet in thickness, of loose 

 fragments. The lower beds generally consist of fine- 

 grained, slightly consolidated tuffs, 1 and the upper beds 

 of great loose fragments, with alternating finer beds. 2 



1 Some of this peperino, or tuff, is sufficiently hard not to be 

 broken by the greatest force of the fingers. 



2 On the northern side of the Green Mountain a thin seam, about 

 an inch in thickness, of compact oxide of iron, extends over a con- 



