CHAPTER XIX.

VOLCANOS AND THEIR PRODUCTS CONTINUED.-LAMINATION OF STREAMS OF LAVA. — LAMINÆ OF SPHERULES IN OBSIDIAN.—COMPOSITION OF VOLCANIC ASHES. VOLCANIC TUFF.—PALAGONITE TUFFS OF ICELAND.-MODIFICATION OF VOLCANIC TUFF BY GASES AND VAPOUR.-SOLUTION OF PALAGONITE TUFF IN ACIDS.-SOLFATARAS.-THE GEYSERS AND THEIR MODE OF ACTION, ICELAND.-SULPHUROUS WATER AND GYPSUM DEPOSITS OF ICELAND.-FUSIBILITY OF VOLCANIC PRODUCTS.-FISSURES IN VOLCANOS FILLED WITH MOLTEN LAVA.-LAVA EJECTED THROUGH FISSURES.DIRECTION OF FISSURES IN VOLCANOS.

ONE kind of lamination observed in igneous rocks has been above noticed (p. 328) as due to the elongation and compression of vesicles, so that by their extreme flattening this structure is produced. In the cases of minerals ejected in an unfused state, the lava current in which they are included moving onwards, so that they would adjust themselves according to their forms and the different velocities of movement produced by friction against the supporting rocks, or any casing of more consolidated portions of the molten stream, we might expect a certain amount of arrangement in planes, or of lamination to be produced. Mixtures of substances of different kinds may sometimes also be so juxtaposed before ejection, that when flowing as a lava current they formed separate layers, the thinner, other circumstances being the same, when the more clongated.* Looking also to the spherical bodies, commonly

*Mr. Darwin (Volcanic Islands, p. 70), when describing the Island of Ascension, enters largely into the causes of lamination in volcanic rocks, seen there and in many other parts of the world. Among other remarks, he concludes "that if, in a mass of cooling volcanic rock, any cause produced in parallel planes a number of minute fissures or zones of less tension (which, from the pent-up vapours, would often be expanded into crenulated air-cavities), the crystallization of the constituent parts, and probably the formation of the concretions, would be superinduced or much favoured in such places; and thus a laminated structure of the kind we are considering would be generated."

The lamination of molten matter is often well exhibited in the slags which have flowed from furnaces, especially in some iron-works.

formed of radiating crystals of part of the compound, observed when glasses are passing into the stony form (examples of which are not unfrequently produced artificially), we should anticipate, under the circumstances of lava passing into the stony from its fluid condition, and movement still prevailing in the mass, that the cooling portions, more especially adjacent to the ground over which the whole was passing, might sometimes have their parts so acted upon that planes, composed of little spherules, might be formed; even alternations of them produced as successive portions of the fluid lava became exposed to similar conditions. Obsidian is but the vitreous state of melted rock, and all the conditions obtaining when artificial glasses are passing into the stony state, such as those producing separate crystals of certain silicates, and the arrangement into spherules, has to be looked for as well in the one as in the other, the modifications depending on the kind and abundance of the different silicates, with due regard to the conditions under which the general mass may have moved or remained quiet. The obsidians, in certain volcanic countries, are especially advantageous for studies of this kind, and will well repay the attention of an observer.* He will also find examples of lamination in volcanic rocks which have passed the vitreous state, or intermixture with that state in cooling, and it will be desirable that such, as well as

Dr. Daubeny points out (Description of Volcanos, p. 256), with respect to the obsidian of Lipari, that "some of its varieties possess a remarkable resemblance to certain products obtained by Mr. Gregory Watt (Philosophical Transactions, 1804) during the cooling of large quantities of basalt, an incipient crystallization beginning to manifest itself in the midst of the vitreous mass in the appearance of white or lighter-coloured spots, which appear to be made up of points radiating from a common centre. In many of the Lipari obsidians, however, these round spots are composed of concentric laminæ, and are disposed in general in lines, so as to give a resemblance of stratification to the mass. In other cases, the whole mass is made up of globules of this kind, which are hollow internally, and are sometimes cemented by black obsidian."

Mr. Darwin gives (Volcanic Islands, p. 54-65) an interesting account of laminated volcanic beds alternating with and passing into obsidian at the Island of Ascension. After describing these beds, he remarks, that "as the compact varieties are quite subordinate to the others, the whole may be considered as laminated or striped. The laminæ, to sum up their characteristics, are either quite straight, or slightly tortuous, or convoluted; they are all parallel to each other, and to the intercalating strata of obsidian; they are generally of extreme thinness: they consist either of an apparently homogeneous, compact rock, striped with different shades of gray and brown colours, or of crystalline felspathic layers in a more or less perfect state of purity, and of different thicknesses, with distinct crystals of glassy felspar placed lengthways, or of very thin layers chiefly composed of minute crystals of quartz and augite, or composed of black and red specks of an augitic mineral and of an oxide of iron, either not crystallized, or imperfectly so." Mr. Darwin also mentions the occurrence of layers of globules or spherulites in the transition of one class of beds into the other, one kind of spherulites white, or translucent, the other dark-brown or opaque, the former distinctly radiated from a centre, the latter more obscurely so.

the obsidians, should be well examined for evidence either of movement while consolidation was being effected, or for the simple and very gradual crystallization of parts during any long period which the whole body of rock may have taken to cool.* In such researches the observer will have to recollect, that the top of a lava stream is so far differently circumstanced from the lower portion, that, while the former is exposed to the atmosphere and all its changes, the latter rests upon a bad conductor of heat, so that somewhat modified effects may often be produced, as regards the arrangement of the component substances, in the one part and the

other.

With regard to the cinder and ash accumulations on the sides of volcanos, the adjacent country, and the far-distant regions to which the latter may be borne, it would be expected that their chemical composition would be similar to the lavas, for the time, of their respective volcanos, should any be thrown out, subject to such modifications as their more complete exposure to the vapours and gases rushing out might occasion. We should anticipate that during the eruptions of trachytic lavas the cinders and ashes would be likewise trachytic, and so with the other kinds of volcanic rocks. Thus, should trachytic have preceded doleritic eruptions, in any localities, the ashes and cinders of the one would have preceded the other. Ashes and cinders being so exposed, particularly the former, to be intermingled with, and surrounded by, these volcanic vapours and gases, much would depend, as to any modification or change in the original mineral substance, upon the time during which this action might last, as also upon the kinds of the vapours

While remarking on the spherulites in obsidians and in artificial glasses, Mr. Darwin calls our attention to the observations of M. Dartigues (Journal de Physique, t. lix, pp. 10, 12, 1804), on the difficulty of remelting spherulitic and devitrified glasses without first pounding them and mixing the whole well together, the separation of certain parts from the general compound in the spherules or crystals rendering this necessary.

*In all such researches the slow cooling of a lava stream has to be well considered. Dr. Daubeny mentions, that he found the temperature of the lava stream, ejected from Vesuvius in August, 1834, to be 330° Fahr., four months after its outflow, the thermometer placed upon the lava, after the scoriæ on the surface had been removed. Daniell's pyrometer gave similar results when introduced into a cavity of the lava (Description of Volcanos, p. 229).

+ M. Dufrénoy (Examen chimique et microscopique de quelques cendres volcaniques; Mémoires pour servir a une Description Géologique de la France, t. iv.) considers that volcanic ashes are most frequently composed of distinct minerals, therein differing from the powder produced by the trituration of rocks, usually formed of the union of several minerals. He therefore infers that volcanic ash "is rather the result of a confused crystallization, produced under the influence of brisk agitation, such as in the saltpetre prepared for the manufacture of gunpowder, than the product of the trituration of lavas in volcanic vents, though the ashes, collectively, do not the less represent the composition of the lava."

and gases to which the ashes or cinders may be exposed. In all cases it would be expected that where the cinders and ashes were the most abundantly and speedily accumulated, as upon the cone or sides of a volcano, the effects arising from an intermixture of the acids and vapours with the ashes and cinders would be the most considerable. For instance, where hydrochloric acid is much mingled with the ashes and cinders, the whole piled around a crater in a hot moist state, such portions as were soluble in that acid might be much acted upon. The like also with sulphurous and carbonic acids.

In considering the original composition and subsequent modification which any mass or layers of volcanic cinders may have sustained, it is also needful for the observer to search for evidence as to the probability of these cinders and ashes having been arranged, as now found, either in the air or beneath water, such, for instance, as is afforded by the occurrence of shells or other organic remains among them, or by layers of detritus or chemically-deposited matter, showing a subaqueous accumulation. Ashes and cinders descending into water, and afterwards arranged by it, would probably be well washed, so that little change would be effected afterwards by any acids adhering to, or mingled with them.

*

The term tuff, or tufa, is not uncommonly given to the ash and cinder accumulations of volcanic regions. Dr. Abich has given the following analyses of the tuff of the Phlegrean Fields, Posilippo, and the Island of Vivara, the two former being termed trachytic tuff, the last basaltic tuff:

1. Yellow tuff, from Nola. 2. Yellow tuff, from Posilippo. 3. White tuff, from Posilippo. 4. Tuff, from Epomoeo. 5. From the crater of Monte Nuovo. 6. Yellow tuff, from the Island of Vivara. 7. Grey tuff, from Vivara.+

*So long since as the time of Sir William Hamilton, shells were detected in the tuff of the vicinity of Naples. They have also been noticed in other localities in that vicinity, and are described as those of species still living.

+ Mr. Dufrenoy (Mémoires pour servir a une Description Géologique de la France, t. iv., p. 384) observes, that the tuffs of Posilippo, Pompeii, and Ischia (the two former analysed by M. Berthier, the last by himself), present nearly the same general characters, with the exception of that of Pompeii, which contains nine per cent. of

Looking at the varied manner in which ashes and cinders may be accumulated, either wholly in the atmosphere or bencath water, to the substances with which they have been mingled in the crater of a volcano, and which may more or less coat or impregnate them afterwards, and to the infiltrations through beds and masses of them subsequently to their deposit, either adding to, abstracting from, or modifying the arrangement of their component substances, we should expect that at times even very solid rocks may be produced, at first sight presenting little of the aspect of an accumulation of fine powder and cinders. Mr. Darwin describes a tuff, apparently of this kind, at Chatham Island (Galapagos Archipelago), one evidently formed at first of cinders and ashes, but now having a somewhat resinous appearance resembling some pitchstones. He attributes this alteration to "a chemical change on small particles of pale and dark-coloured scoriaceous rocks; and this change could be distinctly traced in different stages, round the edge of even the same particle."*

In Iceland, a tuff apparently also in a changed or modified condition from that of its original accumulation, and named palagonitetuff, would seem to be of much importance. According to Professor Bunsen (of Marbourg), the palagonite-tuff of Iceland has a density of 2.43, and contains nearly 17 per cent. of combined

carbonate of lime, a substance which he infers was infiltrated, adding weight to the opinion, that the entombment of Herculaneum and Pompeii was produced by an alluvion of the tuff forming the flanks of Monte Somma, water having greatly aided the filling up of the edifices in the two towns. Remarking on the trachytic tuff of the Phlegrean Fields, Dr. Daubeny observes (Description of Volcanos, p. 16), that the analysis of it proves that, "like pumice, it is only a metamorphosed condition of trachyte." He considers tuff, pumice, and obsidian, as all modifications of the same basis, the two former containing "water chemically combined, namely,-yellow tuff, three atoms; white tuff, two atoms; pumice, one." "Now lava," he continues, "although commonly accompanied at the time of its eruption by abundance of steam, and containing, even for several months afterwards, entangled with it a large quantity of this and other volatile matters, holds no water in chemical combination, so that the fact with respect to tuff and pumice shows, that these formations have been placed under circumstances of another kind than those of molten lavas."

* Volcanic Islands, p. 99. Mr. Darwin describes this tuff, where best characterized, as "of a yellowish-brown colour, translucent, and with a lustre somewhat resembling resin; it is brittle, with an angular, rough, and very irregular fracture, sometimes, however, being slightly granular, and even obscurely crystalline; it can easily be scratched with a knife, yet some points are hard enough just to mark common glass; it fuses with ease into a blackish-green glass. The mass contains numerous broken crystals of olivine and augite, and small particles of black and brown scoriæ: it is often traversed by thin scams of calcareous matter. It generally effects a nodular or concretionary structure. In a hard specimen, this substance would certainly be mistaken for a pale and peculiar variety of pitchstone; but when seen in mass, its stratification, and the numerous layers of fragments of basalt, both angular and rounded, at once render its subaqueous origin evident."

From Palagonia, in Sicily, where a similar tuff is found.