Correspondence — Mr. 0. Poulett Scrope. 441 



them shot up as still liquid drops, and therefore taking the form of 

 "bombs" as they descend; but that these fragments are at first of some 

 considerable size, and only become reduced to the comminution even 

 of the ordinary "puzzolana," in which the grains vary from the size 

 of a nut to that of sea-sand, and ultimately of " impalpable powder 

 or dust," through the efiect of their trituration inter se in the air 

 during the process of repeated falls into, and rejection from, the crater. 

 In proof of this I would adduce the fact, that not only were the 

 fragmentary ejections from the Volcano in the earlier period of the 

 eruption I have named visibly composed for the most part of coarse 

 materials, but that day by day as the explosive bursts from the 

 crater continued, the matter they projected and which fell from the 

 air on the surrounding country, became finer and more pulverulent, 

 until at length towards the close of the eruption after 20 days dura- 

 tion, it consisted solely of a powder so impalpable, that its distinct 

 particles were undiscoverable by aid of the strongest lens I possessed, 

 and which in virtue of this tenuity penetrated the closest apartments 

 in Naples, wherever air could permeate, and lay thick upon the tables 

 and furniture, even where all apertures were carefully closed. 



2. Again, in p. 323, when describing the effects of a submarine 

 eruption, Mr. Forbes says "the molten lava coming in contact with 

 the water, is at once broken up into fragments, coarser or finer in pro- 

 portion to the greater or less cooling power of the water in immediate 

 contact with them, and often in great part instantly converted into 

 fine mud . . . beds of which, spread out by the action of the sea, 

 often enclose shells &c." Now what authority is there for this 

 assertion that the contact of water with molten lava at the bottom 

 of the sea instantly converts it into mud ? It is very difficult, not 

 to say impossible, to do more than guess at the effect produced on a 

 body of lava expelled from a volcanic vent beneath water. But I 

 am strongly inclined to believe that the reduction into coarse ash or 

 fine mud of its superficial portions — for certainly the great bulk of 

 submarine lavas have flowed upon the sea-bottom much in the same 

 way as on dry land, spreading over the lower levels according to the 

 ordinary laws of gravity effecting liquids — has been the result of 

 continued trituration under the influence of the disturbed water, and 

 not of any " instantaneous" division into powder by the sudden con- 

 tact of sea- water. If we are to speculate on the probable effects of 

 a submarine eruption (and we have naturally not many facts to guide 

 us in this matter), it would seem most likely that any outbursts of 

 steam which may take place from an exposed surface of lava, in such 

 a position, would, if the depth of water above it were considerable — 

 say 100 feet or more — be condensed long before they could reach the 

 upper air. But these bursts may very probably drive upwards scoriae 

 and ash, that is tattered fragments of lava torn from its surface, just 

 as when the eruption is sub-aerial. Such a jet of fragmentary lava 

 would scarcely rise above the sea-level until the apex of the sub- 

 marine volcano had been raised by the accumulation of ejected matter 

 to within so short a distance of the surface as to permit the bursts of 

 steam, uncondensed, together with the fragmentary matters they drive 



