Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/260

Rh 246 at the rate of about 7 miles an hour in an opposite direction to that of the wind which blew at the surface. On several occasions ashes from one of the Icelandic volcanoes have fallen so thickly between the Orkney and Shetland Islands that vessels passing there have had the unwonted deposit shovelled off their decks in the morning. In the year 1783, during an eruption of Skaptar-Jiikull, so vast an amount of ﬁne dust was ejected that the atmosphere over Iceland con- tinued loaded with it for months afterwards. It fell in such quantity over parts of Caithness—a distance of 60 miles——as to destroy the crops; that year is still spoken of by the inhabitants as the year of “ the ashie.” Traces of the same deposit were observed even as far as Holland. Hence it is evident that volcanic deposits may be formed in regions many hundreds of miles distant from any active volcano. A single thin layer of volcanic detritus in a group of sedimentary strata would thus not of itself prove the existence of contemporaneous volcanic action in its neigh- bourhoad. It might be held to have been wind—borne from a volcano in a distant and separate region. Lava-streams.—A microscopic examination of their in- timate structure shows that lavas have been truly molten rocks. They usually consist fundamentally of a glass through which are diffused, in greater or less abundance, various microlites and crystals. Their degree of liquidity, at the time of emission, seems to depend on the extent to which the rock remains in the condition of glass, viscidity increasing with the development of the microlites and crystals out of the glassy menstruum in which, no doubt, originally their component molecules were diffused. The ﬂuidity may also be governed in no small degree by the amount of vapour existing interstitially in the molten mass. Mr Scrope indeed contended that aqueous vapour was the main cause of the mobility of such crystalline lavas as those of Vesuvius. But even where the lava pours forth with a liquidity like that of melted iron, it speedily assumes a more viscid motion, as the process of devitriﬁcation advances and the rock is exposed to the chilling effects of radiation and of contact with air and soil. An interesting fact, admirably shown by the micro- scope, but often easily observable with the naked eye, is that in lava still liquid and mobile well-deﬁned crystals make their appearance. These sometimes are broken during the continued movement of the surrounding mass, the separated fragments becoming involved in the general glassy base or portions of that base, are injected into the fractures of the crystals. ‘Veil-defined crystals of leucite may be seen in specimens of Vesuvian lava, which has been ladled out from a white-hot stream, impressed with a stamp, and thus suddenly congealed. On the other hand, the obsidians have solidiﬁed in the condition of complete glass, often without any trace of devitrifieation. The green pyroxenic lava of Hawaii exhibits so extreme a degree of ﬂuidity that, during its ebullition in pools of the crater, jets not more than a quarter of an inch in diameter are tossed up, and, falling back on one another, make “ a column of hardened tears of lava,” while, in other places, the jets thrown up and blown aside by the wind give rise to long threads of glass which lie thickly together like mown grass, and are known by the natives under the name of Pele’s Hair, after one of their divinities} It would be of the highest interest and importance to know accurately the temperature with which a lava stream issues. The difﬁculty of making any direct observation at the point of outﬂow has hitherto been insuperable. Measurements have been taken at various distances below the point where the moving lava could be safely approached ; but these are not satisfactory, seeing that the outer crust of 1 Dana, Geol. US. Erplor. E.r_'pcd., p. 179. GEOLOGY [uI. Dv:'.-i.c-tL. the lava cools rapidly, and gives no measure of the tempera- ture even a short way underneath. Experiments made by Scacchi and Sainte-Claire Deville on the Vesuvian lava erupted in 1855, by thrusting thin wires of silver, iron, and copper into the lava, indicated a temperature of scarcely 700° C. Earlier observations of a. similar kind, made in 1819, when a silver wire —,,‘Gth inch in diameter at once melted in the Vesuvian lava of that year, gave a greatly higher temperature. Evidence of the high temperature of lava has been adduced from the alteration it has effected upon refractory substances in its progress, as where, at Torre del Greco, it overtlowed the houses, and was after- wards found to have fused the ﬁne edges of ﬂints, to have decomposed brass into its component metals, the copper actually crystallizing, and to have melted silver, and even sublimed it into small octohedral crystals. But s11ch f u-t-, though full of interest and importance, give us no clue to the absolute initial temperature of the lava, which must be greatly higher than that of the stream after several miles of descent on the mountain slopes, and after some hours or days of cooling. In spite of this very high temperature, however, the lava issues abundantly charged with aqueous vapour, to the expansion of which, as we have seen, its ebullition and ex- pulsion are mainly due. As this vapour at once begins to escape when the lava issues into the air, it shows itself by a dense white cloud hanging over the moving mass. The lava streams of Vesuvius sometimes appear with as large and dense a steam cloud at their lower ends as that which escapes at the same time from the main crater. Even after the molten mass has ﬂowed several miles, steam continues to rise abundantly both from its end and from numerous points along its surface. From the wide extent of basalt dykes, such as those of Britain, some of which rise to the surface at a distance of 200 miles and upwards from the main volcanic regions of their time, it is evident that the molten lava may sometimes occupy a far greater superﬁcial area under- neath than the mere circumference of the actual pipe or of the volcanic cone. We must conceive of avast reser- voir of melted rock impregnated with superheated steam, and impelled upwards by the elastic force of the vapour. The lava may be regarded rather as the sign than as the cause of volcanic action. It is the pressure of the impris- oned vapour, and its struggles to get free, which produce the subterranean earthquakes, the explosions, and the outpour- ing of lava. As soon as the vapour ﬁnds relief, the terres- trial commotion calms down again, and the quiescence con- tinues until another accumulation of vapour demands a re- petition of the same phenomena. It is evident that the vapour may succeed in effecting its escape without driving molten rock up to the surface. There may be tremendous explosions without an actual outcome of lava. But, in most cases, so intimately are vapours and lava commingled in the subterranean reservoirs that they rise together, and the explosions of the one lead to the outflow of the other. The first point at which the lava makes its appearance at the surface will largely de- pend upon the structure of the ground. Two causes have been assigned in a foregoing section (p. 244) for the fissur- ing of a volcanic cone. As the molten mass rises within the chimney of the volcano, continued explosions of vapour take place from its upper surface, the violence of which may be inferred from the vast clouds of steam, of ashes, and of stones which are hurled to so great a height into the air. These explosions must at the same time powerfully affect the sides of the funnel, exposed as these are to the enormous pressure exerted by the imprisoned vapour. We cannot therefore be surprised that, when a volcano experiences shocks of such intensity as to be felt over a radius 100