46 Pmceediufjx of !/'<' lio'jal Socidfj of Victorln. 



asserted they only occur on the surface, and have not been 

 found vvitliin the conglamerates. Their source is not mentioned, 

 so that it is not quite certain wliether they have been established 

 as true bombs. 



Stelzner also describes an obsidian Iwmb from Mexico, given 

 to him by Rosenbusch, which he compares with obsidianites, but 

 does not think the resemblance of form very close. Unfortunately 

 he gives no particulars of its occurrence or its internal structure, 

 so that we are left in doubt on these points. Darwin, in 

 comparing these objects with the bombs from Ascension Island, 

 was evidently misled, from the examination of a perfect specimen, 

 l)y the pittings,- which bear a strong resemblance to external 

 signs of vesicularity ; and Clarke also mentions them as being 

 similar. The hollow obsidianite can scarcely be called vesiculai-, 

 as the wall, with the exception of a few isolated vesicles, is 

 compact. The .smooth, polished interior is sufficient evidence of 

 this. Let us now view the operation by which bombs are 

 originated, and see whether it favours a solid or vesicular 

 character. 



Professor Judd^ gives a graphic description of the phenomena 

 taking place in an active volcano vent, and I cannot do better 

 than repeat it here. It is as follows : — " If we take a tall, 

 narrow vessel and fill it with porridge, or some similar substance 

 of imperfect fluidity, we shall be able, by placing it over a fire, 

 to imitate very closely indeed the appearances presented in the 

 crater of Stromboli. As the temperature of the mass rises, steam 

 is generated within it, and in the efforts of this steam to escape 

 the substance is set in violent motion. These movements of the 

 mass ai'e partly rotatory and partly vertical in their direction ; 

 as fresh steam is generated in the mass its surface is gradually 

 raised, while an escape of the steam is immediately followed by a 

 fall of the surface. Then an up and down movement of the 

 liquid is maintained, but as the generation of steam goes on 

 faster than it can escape through the viscid mass, there is a 

 constant tendency in the latter to rise towards the mouth of the 

 vessel. At last, as we know, if heat continues to be applied to 

 the vessel, the fluid contents will be forced up to its edge, and a 



1 Volcanoes : What they are and what they teach. 1885 ed., p. 20. 



