Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/265

Rh VOLC.-NOES.] many districts of northern Italy, at Tamar and Kertch, at Baku on the Caspian, over an area of about 1000 square miles near the mouth of the Indus, and in other parts of the globe. Cr'r1se0us Disc/Largjes.-—S0me of these belong to true volcanic phenomena, others are closely associated with the u1ud—volcauoes. To the former class we may assign the copious emanations of carbonic acid which so frequently take place in districts where volcanic activity has been long dormant or extinct. The gas either comes out directly f ro1n ﬁssures of the rock, or rises dissolved in the water of springs. The old volcanic districts of Europe furnish many examples. Thus on the shores of the Laacher See—an ancient crater lake of the Eife1—carbonic acid gas issues from numerous openings called 7no_{i'e¢‘tr’, round which dead insects, and occasionally mice and birds may be found. In the, same region occur hundreds of springs more or less charged with the gas. The famous Valley of Death in Java contains one of the most remarkable gas-springs in the world. It is a deep, bosky hollow, from one small space on the bottom of which carbonic acid issues so copiously as to form the lower stratum of the atmosphere. Tigers, deer, and wild-boar, enticed by the shelter of the spot, descend and are speedily suffocated. Many of their skeletons, together with those of man himself, have been observed. In the second class of gas-springs we may group the emanations of carburetted hydrogen, which, when they take ﬁre, are known as Fire-wells. They occur in many of the districts where mud-volcanoes appear, as in northern Italy, on the Caspian, in Mesopotomia, in southern Kurdistan, and in many parts of the United States. It has been observed that they rise especially in regions where beds of rock-salt lie underneath, and as that rock has been ascertained often to contain compressed carburetted hydrogen, the solution of the rock by subterranean water, and the consequent liberation of the gas, has been offered as an explanation of these ﬁre-wells. G'r'_y.se7's.—I11 various regions where volcanic action still continues, or where it has long been dormant, there occur eruptive fountains of hot water and steam, to which the general name of geysers is given, from the well-known examples in Iceland, which were the first to be seen and described. Besides the Great and Little Geysers, the Strokkr, and other minor springs of hot water in Iceland, other, perhaps still more striking, examples have in recent years been brought to light in that tract of the western territories of the United States set aside as the “Yellow- stone National Park,” and good illustrations are also found in New Zealand. A geyser possesses a vertical pipe in the ground, terminating at the surface in a basin which is formed of siliceous sinter, and may rise some feet or yards above the general level. At more or less regular intervals rumblings and sharp detonations occur underneath, followed by an agitation of the water in the basin, and then by the violent expulsion of a column of water and steam to a con- siderable height in the air. The hot water contains silica in solution, which, on cooling and evaporating, is deposited at the surface; and thus the geyser builds up its basin, sometimes raising it into a long, solitary, ﬁnger-like pillar. Bunsen and Descloiseaux spent some days experimenting at the Icelandic geysers, and ascertained that in the Great Geyser, while the surface temperature is about 212° Fahr, that of the lower portions of the tube is much higher—a ther- mometer giving as high a reading as 266° Fahr. The water there must consequently be 48° above the normal boiling- point, but is kept in the ﬂuid state by the pressure of the overlying column. At the basin, however, the water cools quickly. After an explosion it accumulates there, and eventually begins to boil. The pressure on the column below being thus relieved, a portion of the superheated GEOLOGY 251 water ﬂashes into steam, and as the change passes down the pipe, the whole column of water and steam rushes out with great violence. The water thereafter gradually collects again in the pipe, and after an interval of some hours the opera- tion is renewed. The experiments made by Bunsen proved the cause of the eruption to lie iii the high temperature of a portion of the pipe. He hung stones by strings to different depths in the funnel of the geyser, and found that only those in the higher part were cast out by the rush of water, sometimes to a height of 100 feet, while at the same time the water at the bottom was hardly disturbed at all.1 These observations give an additional interest and im- portance to the phenomena of geysers in relation to those of volcanic action. They show that the eruptive force is steam ; that the water column, even at a comparatively small depth, has a temperature considerably above 212'’ ; that this high temperature is local 5 and that the eruptions of steam and water take place periodically, and with such vigour as to eject large stones to a height of 100 feet. § 3. Structure of Volcanoes. It is now admitted that a volcano is due to the accumu- lation of material round the vent of eruption, and not to any blister-like expansion of the ground. The structure of a volcanic cone necessarily depends in great measure upon the nature of the substances ejected. The following are the more important and interesting types of this kind of structure :—- (l.) Cones of Non-v0lccmz'c J[ale2'2'als.—These are due to the discharge of steam or other aeriform product through the solid crust without the emission of any true ashes or lava. The materials ejected from the cavity are wholly, or almost wholly, parts of the surrounding rocks through which the volcanic pipe has been drilled. Some of the cones sur- rounding the crater-lakes or maare of the Eifel consist chieﬂy of fragments of the underlying Devonian slates. (2.) Tzgf-Cones, CinderC'o2zes.—Successive eruptions of ﬁne dust and stones, often rendered pasty by getting mixed with the water so copiously condensed during an eruption, form a co11e in which the materials are solidiﬁed by pres- pure into (puff. 1»_SLomeatIimes t:lD1Ie cones apiel mﬁde upfoiﬁy of oose cm ers, 1 'e I onte uovo in e ay 0 aiae. Cones consisting entirely of loose volcanic materials often arise on the ﬂanks or round the roots of a great volcano, as happens to a small extent on Vesuvius, and on a larger scale upon Etna. They likewise occur by themselves apart from any lava—producing volcano, though usually they afford indications that columns of lava have risen in their funnels, and even now and then that this lava has reached the surface. Admirable exam les are furnished by the cones of the Phlegraean ﬁelds nezii Naples. Ancient cones of a similar character occur among the Carboniferous rocks of Scotland. The materials of the cone are arranged in more or less regular beds which dip away from the funnel, their inclination corres )onding with that of the cone. Inside the crater they slope lsteeply inward towards the crater-bottom. (3.) J[ucl—Cones or Salscs are formed by the accumulation and consolidation of mud round the vents of mud-volcanoes. They sometimes reach a height of 400 feet. (4.) Lava-C'ones.—These are comparatively rare, since, in most cases, the emission of lava is accompanied by the discharge of ashes. Owing to its liquidity, the lava ﬂows off quickly, and the cones have very gentle slopes. The most remarkable examples are those in the Hawaii Islands described by Professor Dana. They attain a great height, but so small is their angle of inclination, that they may be described as only gently—sloping mounds, and their craters have been compared to vast open quarries on a hill or moor. 1 Bunsen, Ann. (lcr Chcmie and P/zar7nacz'e._ lxii. (18-17), p. 1.