no 



NA TURE 



\_Nov. 29, 188S 



within each block expands, due to the liberation, as 

 vapour, of the dissolved. HgO, and the formation of a 

 vesicular structure, which may progress to such a point 

 as to constitute a true pumice. This is accompanied by 

 fissuring of the external hardened surface, just as the 

 expansion of dough splits open the crust, as the air- 

 bubbles expand before and after the loaf is in the oven — 

 in fact, we could not adopt a better term to define this 

 structure than bread-crust structure. These fissures often 

 divide crystals, pieces of included rock, &c., showing that 

 little plasticity was left on the surface when this expansion 

 took place. In fact, the conditions necessary are that the 

 glass be sufficiently cold to break with a strain or blow 

 applied sharply, but to bend when the force employed is 

 gradual in its action, such as may be seen well in all 

 vitreous substances. If we warm a stick of seahng-wax, 

 hardened Canada balsam, &c., we may gently bend it to 

 any form, but if our attempt is too quick, the stick breaks. 

 In these iDombs many of the surfaces appear to have first 

 bulged and then broken. As these are ejected, and con- 

 sist of a hard crust and soft interior, I suppose we must 

 use that unfortunate term bomb, though they are rarely 

 round and certainly do not strike one as resembling as 

 much a bomb as do those masses found on the surface of 

 lava-streams. 



The colour is buffish-gray, the surface someAvhat glisten- 

 ing and scattered over by the exposed broken surfaces 

 (split by the division fractures between the contiguous 

 blocks) of a dark green mineral, chiefly pyroxene, glassy 

 crystals of feldspar, smaller black metallic lustred grains 

 of magnetite, more rarely typical grainsof olivine, quartz, 

 and pyrites. There are also very many grains of different 

 sizes of a darker-coloured fine-grained rock which in- 

 closes- many of the augites, feldspars, olivines. Micro- 

 scopically, the vesicular structure is seen to extend, though 

 becoming less marked, to within (in the specimen ex- 

 amined) less than a millimetre of the surface, though in 

 larger blocks from other eruptions preserved in my 

 collection this may attain 2 centimetres or more. The 

 crystals of pyroxene are usually well formed, though often 

 broken. They include, wholly or partly, large, rather 

 irregular magnetites, and in some cases are surrounded 

 by wreaths of either sanidine or, more commonly, triclinic 

 feldspar, probably labradorite. Where included in foreign 

 rock-fragments, this latter is seen to be composed of a 

 network of magnetite, augite, and feldspar microliths, and 

 is often much altered. The feldspars are principally 

 sanidines which may attain half a centimetre long ; they 

 are very dirty from inclusions, and somewhat rounded. 

 There are also groups of labradorite crystals, and another 

 triclinic feldspar in which the striations are remarkably 

 close and fine. In some cases a triclinic feldspar seems 

 to be intergrown with the sanidine. Of the latter mineral 

 there are many microcrystals and microliths. Here 

 and there are to be met with a few ill-formed crystals of 

 dark-green amphibole. 



What part of these minerals belong to the essential 

 magma, and what are simply imperfectly fused out of the 

 surrounding rock, it is extremely difficult to determine, 

 and chemical analysis of the rock would be obviously 

 useless, on account of the numerous inclusions of other 

 rock-fragments. The association of such basic minerals 

 with a distinctly acid rock would be very remarkable, 

 were it not for the distinct origin of them by inclusion of 

 accessory materials. The eruptive rocks of this island 

 range from a very rich olivine basalt through a dolerite 

 to the typical obsidians and spherulites. There is little 

 doubt that these included minerals are the churned-up 

 fragments in the crater apex which almost certainly cuts 

 through those older rocks, and even part of the present 

 active cone of Vulcano is composed of dolerite. 



That these bombs are the primary ejectamenta in this 

 eruption there is no doubt, on account of their freshness 

 and the sharp uneroded angles and edges, as observed by 



Signor Platania, together with the absence of any solfata- 

 rizing. The specimen examined was ejected during the 

 month of August, probably early in that month. 



The next specimen is dated August 18, and consists of 

 coarse sand or fine lapilli, about the size of a mustard- 

 seed, with a little fine gray ash. This I made into an 

 artificial breccia, and cut sections of it. It is compcsed 

 of broken fragments of dolerite and glassy rocks, both 

 often solfatarized, with chips of pyroxene, magnetite, &c., 

 and, no doubt, is chiefly accessory ejectamenta derived 

 from the crumbling sides of the crakter being churned, 

 ground up, and ejected. 



Next is a fine ash of light gray colour, ejected on 

 August 26, which is, in great part, also composed of 

 similar materials to the last, with an abundance of very 

 minute microliths, many of a dark-green colour, and 

 therefore probably pyroxene or amphibole, though they 

 remain dark between crossed nicols, from their great 

 minuteness. I have observed no trace of tridymite found 

 in such abundance in the ash of one of the recent erup- 

 tions of Vulcano. This we should expect to be formed at 

 a later date, when the more tranquil vapours, escaping 

 through the material at the crater bottom, would allow of 

 their deposition. 



From the description of the eruption by Mr. Narjian 

 that was given in my paper on this subject at the British 

 Association meeting this year, and reproduced in the 

 Times and other newspapers, from the examination of the 

 eruptive products, and from the state of the volcano pre- 

 vious to its last eruption (see my paper, " The Islands of 

 Vulcano and Stromboli," Nature, vol. xxxviii. p. 13) 

 taken together with what we learn from the ejectamenta, 

 we may obtain a fair idea of the eruptive process in this 

 case. The chimney of the volcano was, no doubt, filled 

 by an acid magma, which, perhaps, after the last eruption, 

 was of much higher temperature, and in which fragments 

 of other rocks from the crater and chimney sides had 

 been churned up and partially fused. The temperature 

 and liquidity seem to have been low, as the olivines and 

 augites, although they have apparently been fused out of 

 their original matrix, especially the latter, retain most 

 perfectly their crystalline angles, and no chemical fluxing 

 or reaction seems to have occurred between the basic 

 minerals and the surrounding acid magma. Also, the 

 occurrence of pyrites points in a similar manner to 

 the same physical state. The choking of the crater after 

 the former eruption, together with the gradual cooling 

 of the upper part of the magma column during the inter- 

 mediate solfataric stage of the volcano, would result in a 

 gradually increasing obstacle to the boiling-off of the 

 H.,0 dissolved by the magma lower down. Two pro- 

 cesses would therefore be going on, viz. increased super- 

 incumbent pressure, and augmenting tension of the part 

 of the subjacent magma within reach of water-supply. 

 The latter must obviously, after a certain time, increase 

 in a greater ratio than the former, until the plug is blown 

 asunder. 



This plug in great part would consist of the magma 

 with its inclusions reduced to that critical state between 

 a liquid and solid, as seen in vitreous bodies. When this 

 is broken up by the sudden impulse of the expansion of 

 the subjacent aquiferous magma, it would split into frag- 

 ments ; and, these immediately cooling on their surface by 

 the molecular formation and escape of vapour near that 

 surface, cooling and solidification would result, but 

 before this extended far in, the hotter interior would 

 undergo frothing, and so bend, crack, and fissure the 

 nearly hard coating, producing in this manner the 

 bread-crust structure. These blocks seem from Mr. 

 Narlian's account, to have fallen nearly red hot, as his 

 children's feet were burnt, and part of the house where 

 they fell was burnt. After the first explosion, a series 

 of feebler explosions took place, and, I believe, are 

 still continuing with diminished force, just as is seen 



