Obsidian Boinbs in Aastndia. 47 



catastrophe will occur, the steam being suddenly and violently 

 liberated from the bubbles formed on the surface of the mass, 

 and a considerable quantity of the material forcibly expelled 

 from the vessel." 



Under these conditions the surface of the molten nnass must 

 then be constantly traversed by jets of escaping steam, until, as 

 is pointed out, the steam which collects faster than it can be 

 liberated in this manner, frees itself by an explosion of greater 

 or less intensity, carrying with it fragments of the lava. Now 

 this explosion takes place near the surface in the region of 

 the lava, which must necessarily be permeated with steam and 

 consequently in a vesicular condition ; it is thereby broken up 

 and partly ejected in the form of vesicular bombs, lapilli and 

 dust. The steam imprisoned in the bombs, during their passage 

 through the air, tends to expand, and they become more or less 

 completely distended with bubbles. This action must be the 

 same no matter what the nature of the lava may be, only that 

 probably in the case of acid lavas the explosive action might be 

 much more violent, owing to their greater viscosity permitting 

 a larger quantity of steam to accumulate and become more 

 highly heated before being expelled. This, then, may have had 

 Some influence upon the apparent scarcity of acid bombs ; that 

 is to say, the lava in this instance wovild not be sufficiently fluid, 

 when ejected, to take any very regular form, and would appear 

 more as fragmentary pumice. The difference in the liquidity 

 is evidenced by the fact that basic lavas form much more 

 extensive sheets, sometimes flowing to great distances from the 

 vents, than do the acid lavas, which tend rather to accumulate 

 round the points of eruption. From what has been said it will 

 be seen that the origin of volcanic bombs is of such a character 

 as to reasonably infer that they would mostly be of a vesicular 

 nature, whether acid or basic, and the apparent absence of 

 obsidian bombs in the North Island of New Zealand, and in fact 

 of all solid bombs in that and this colony as well, strengthens 

 the idea. 



The forms presented by the obsidianites are so extraordinary 

 and curious that they at once direct attention as being a most 

 unusual natural occurrence. As far as I can gather, most of 

 these forms have not an analogy amongst volcanic ejectamenta, 



