Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/256

Rh 242 structure of true lavas. As a rule it betrays its aqueous origin by more or less distinct evidence of stratiﬁcation, by the multifarious pebbles, stones, blocks of rock, tree-trunks, branches, shells, bones, skeletons, which it has swept along in its course and preserved within its mass. Sections of this compacted tuﬂ‘ may be seen at Herculaneum. The trass of the Brohl Tlial and other valleys in the Eifel district, referred to on p. 239, is another example of an ancient volcanic mud. 2. Lavas.—The term lava is applied generally to all the molten rocks which ﬂow out from volcanoes} The behaviour of the lava as it issues and flows down the volcanic cones will be described in the next sub-section. It will be suﬂicient to refer here to some of the leading pecu- liarities of the lavas viewed as volcanic products. (1.) Their speciﬁc gravity ranges between 2'37 and 3'22. (2.) The heavier varieties contain much magnetic or titani- ferous iron, with augite and olivine, their composition being basic, and their proportion of silica averaging about 45 or 50 per cent. In this group come the basalts, dolerites, nepheliiie-lavas, and leucite-lavas. The lighter varieties contain commonly a minor proportion of metallic bases, but are rich in silica, their percentage of that acid ranging between 60 and 80. They are thus not basic but acid rocks. Among their more important species, trachyte, rhyolite, obsidian, pitchstone, and pumice may be enu- merated. They diﬁer much in texture. ((1) Some are entirely crystalline, consisting of nothing but an interlaced mass of crystals and crystalline particles, as in some dolerites. (b) Some show more or less of a half-glassy or stony matrix, in which the constituent crystals are inibedded ; this is the most common arrange- ment. (c) Others are entirely vitreous, such crystals or crystalline particles as occur in them being quite subordi- nate, and, so to speak, accidental enclosures in the main glassy mass. Obsidian or volcanic glass is the type of this group. (cl) They further differ in the extent to which their mass has been affected by the interstitial steam at the time of their eruption. Some show no outward signs of any inﬂuence from that cause ; in others, however, the expansion of the imprisoned steam has been such as to expand the still molten stone into an open cellular texture, somewhat like that of open ill-baked bread. Such a vesicular arrange- ment very commonly appears on the upper surface of a lava current. They vary greatly in colour and general external aspect. The heavy basic lavas are usually dark- grey, or almost black, though, on exposure to the weather, they usually acquire a brown tint from the oxidation and hydration of their iron. Their surface is commonly rough and rugged, until it has been suﬂiciently decomposed by the atmosphere to crumble into excellent soil which, under favourable circumstances, supports a luxuriant vegetation. The less dense lavas, such as phoiiolites and trachytes, are frequently paler in colour, sometimes pale yellow or buﬁ", and decompose into light soils; but the obsidians present rugged black sheets of rock roughened with ridges and heaps of froth-like pumice. 3. Fragmcntary 1l[aterials.——Uiider this title we include all the_ substances which, driven up into the air by volcanic explosions, fall in solid form to the ground—-the dust, ashes, sand, cindcrs, and blocks of every kind which are projected from a volcanic oriﬁce. These materials must obviously differ greatly in composition, texture, and appear- ance, even during a single eruption, and still more in suc- cessiveexplosions of the same volcano. For the sake of convenience separate names are applied to some of the more 1 “ Alles ist lava was im Vulkane ﬂiesst uiid (lurch seine Fliissigkeit neue'I_.agerst;'ttter einnimmt” is Leopold Von Buch's comprehensive deﬁnition. GEOLOGY [iii. DYNAMICAL. distinct varieties. Of these the more important are the following. Ashes and Saml.——In niany eruptions vast quantities of an exceedingly ﬁne light grey powder are ejected. As this substance greatly resembles what is left after a piece of wood or coal is burnt in an open ﬁre, it has been popularly termed ash, and this uaine has been adopted by geologists. If, however, by the word ash the result of combustion is implied, its employment to denote any pro- duct of volcanic action imist be regretted as apt to convey a wrong impression. The fine aslilike dust ejected by a volcano is merely lava in an extremely fine state of com. niinution. So minute are the particles that they ﬁml their way readily through the ﬁnest chinks of a closed room, and settle down upon ﬂoor and furniture as ordinary dust does when a house is shut up for a short time. I‘i'oiii this ﬁnest form of material gradations may be traced through what is termed volcanic sand into the coarse varieties of ejected matter. In composition the ash and sand vary necessarily with the nature of the lava from which they are derived. When the fragments range from the size of a pea to that of a walnut they are called lapilli. These are pieces of lava, round, subangular, or angular in shape, and having the same indeﬁnite range of coniposition as the ﬁner dust, As a rule, the coarse fragments fall nearest the focus of eruption. Sometimes they are solid fragments of lava, but more usually they have a cellular texture, while soiiie- times they are so light and porous as to ﬂoat readily on water, and, when ejected near the sea, to cover its surface over a large area. Volca-m'c Blocks are large pieces of stone, often angular in shape. In some cases they appear to be fragments loosened from already solidiﬁed rocks in the chimney of the volcano. Hence we ﬁnd among them pieces of older tuffs, and of lavas recogiiizably belonging to early eruptions. But in other and iuiiiierous instances they are not volcanic rocks at all, but belong to earlier forma- tions through which the volcanic chiiiuiey has been drilled. Blocks of a coarsely crystalline granitoid lava have been par- ticularly observed both on Etna and Vesuvius. In the year 1870 a mass of that kind weighing several tens was to be seen lying at the foot of Vesuvius, within the entrance to the Atrio del Cavallo. Similar blocks occur among the Car- boniferous volcanic pipes of central Scotland. In the older tuffs of Soinma, blocks of altered limestone form the chief repositories of the Vesuviaii minerals. (4.) Volcanic L'07)l,JS and Slags.—These have originally formed portions of the column of lava ascending the pipe of the volcano, and have been detached and hurled into the air by the successive explosions of steam. In the case of a bomb, we have a round, elliptical, or pear-shaped and often discoidal mass of lava, from a few inches to several feet in diameter. Sometimes it is tolerably solid throughout, more usually vesicular. N ot uiifrequcntly it consists of a hollow interior, with a shell which is vesicular towards the centre and becomes more close-grained towards the outside. There can be no doubt that, when torn by eructations of steam from the surface of the boiling lava, the material of these bonibs was in as thoroughly iiiolten a condition as the rest of the mass. From the rotatory inotion imparted by its ejection, it took a circular form, and in proportion to its rapidity of rotation and ﬂuidity would be the amount of its “ ﬂattening at the poles.” The centrifugal force within would allow the expansion of the interstitial vapour, while the outer surface would rapidly cool and solidify ; hence the solid crust, and the porous or even cavernous interior. Such bombs, varying from the size of an apple to that of a 1iian’s body, were found by Mr Darwin abundantly strewed over the ground in the island of Ascension? They have been found likevise in the tuﬂ? of 9 G'col0_qz'ca.l Obser-vatious on Vulcaizic Islands, 2d edit., p. 42.