736 



DYNAMICAL GEOLOGY. 



Other Volcanic Phenomena. 



(1.) In submarine eruptions, islands have been made by the ejections. Such islands 

 may consist of lava below, but are certain to be of scoria, conglomerate, and tufa above 

 through the action of the cold waters on the lava, if not so from cinder ejections. An 

 island named Graham Island, was thus made in a few days in 1831, off the south coast 

 of Sicily. It had a height above its base of 800 feet, 200 of this above the sea-level ; 

 but by the close of the year it had disappeared through the wear of the sea, leaving a 

 shoal in its place. 



(2.) Volcanic bombs are thrown from some craters when the lavas are rather viscid. 

 They have usually a centre of grains or masses of infusible material, which the movement 

 in the lavas has rolled together into a mass ; the indigestible cud is ejected with the 

 cinders, and becomes further rounded in the rotation through the air. Such bombs are 

 sometimes twelve or fifteen feet in diameter. At the Eifel, in Prussia, a region of ex- 

 tinct volcanoes, the interior is mainly chrysolite ; and at the island of Lipari, it is feld- 

 spathic scoria, with some chrysolite. 



(3.) A circulating movement, like that of a boiling cauldron, has been observed in the 

 lava of Kilauea. In the lava-conduit, the greatest heat is along the centre, most re- 

 mote from the cold sides. Hence the ascent from inflation by rising vapors would be 

 greatest at the centre, which should determine a flow at surface from the centre to the 

 sides. This was exhibited on a grand scale at Kilauea, in 1840, where the liquid lava 

 in the great lake 1,000 feet across (a, Fig. 1112, p. 725), seemed like a river that came 

 to the surface for a moment and then disappeared. The area of greatest heat was near 

 the northeast side of the lake ; and the stream seemed to flow to the southwest. 



(4.) The slope of flowing lava, while generally small and producing cones of small 

 angle, may be of almost any angle. It forms continuous streams of 30°; and even ver- 

 tical cascades of solid lava occur about Mount Loa and other volcanoes. As Prevost 

 observed, flowing lava, like flowing beeswax, if stream follow stream rather rapidly, 

 and not too copiously, so that one becomes melted to another, may make layers of great 

 thickness, having a large angle of inclination. Hence, while the average angle of a 

 lava-cone is small, because lavas when in a very large outflow spread rapidly and easily, 

 there are many regions of much steeper angle, over its declivities. The author observed 

 a stream descending into the crater of Kilauea, at an angle of 30°. It was, however, 

 hollow, the interior having run out after the crust had formed. Mr. Coan mentions the 

 frequent occurrence of slopes of 15° to 20° and more, along the stream formed at the 

 eruption of Mount. Loa in 1855. Columns of lava-drops rising from a spreading base, 

 looking like petrified fountains, are formed, when small jets of lava are thrown verti- 

 cally from a hole, and fall back on one another before cooling. The stony drops are 

 sometimes less than half an inch across, and thus evince the remarkable liquidity of the 

 lavas. 



(5.) Lavas cool at surface suddenly owing to contact with the air, and make thus a 

 crust a foot to a yard thick; but underneath this crust of poorly conducting material, 

 the further cooling goes on with extreme slowness, varying in rate according to the 

 thickness of the mass. The liquid part below often flows on and leaves open chambers 

 or long passages. 



(6.) The texture of the rock made by solidification is finegrained or coarse-grained, 

 contains much glassy material, or little or none, according to the rate of cooling, and 

 some other conditions, especially the not too free supply of moisture within the lava. 

 In the final cooling of the interior of a volcano, the rock is sometimes granite-like in its 

 coarse crystallization owing to the slowness of the cooling process. 



(7.) Solfataras. — Solfataras are areas where sulphur-vapors escape, 

 and sulphur-incrustations form. They occur away from intense vol- 

 canic action, where sulphur vapors and steam rise slowly. Incrusta- 

 tions of alum are common in such places, arising from the action of 

 sulphuric acid on the alumina and alkali of the lavas. The decompo- 



