Volcanos. 129 



other component beds, acts as braces or ties to the frame-work, 

 and increase its general solidity. Somma presents an example 

 of such dikes in great numbers. The fissures are in this manner 

 hermetically sealed, as it were, and never open a second time. 

 Thus, with the height and bulk, the strength of the mountain in- 

 creases, without any conceivable limit ; and the author thinks 

 it an erroneous idea, that such limit exists, and that, at a certain 

 height, eruptions can no longer take place from the summit of 

 the cone, since every lateral eruption adds to the strength of 

 the mountain's flanks, and to the resistance they oppose to the 

 lateral pressure of the internal column of lava. Parasitic cones 

 are thrown up by the gaseous explosions which take place from 

 these lateral vents. They have each their crater, and each 

 marks the source of a current of lava. iEtna has nearly seventy 

 such cones scattered on its flanks, many of considerable size. 

 Vesuvius exhibits but a few, but is itself a parasitic cone, thrown 

 up in the centre of the old crater of Somma. The skirts and 

 base of a volcanic mountain are usually covered with conglome- 

 rates of an alluvial character, deposited by torrents of water, 

 proceeding either from the violent rains, which usually follow 

 an eruption, or from the melting of snows. These debacles of 

 mujl and water, are called by the inhabitants of Vesuvius, lave 

 aVacqua. In Iceland, they constitute the most destructive part of 

 the volcanic phenomena. Such deposits are often carried to 

 some distance from the foot of the mountain, and are found al- 

 ternating with the currents of lava which have flowed farthest 

 from the centre of eruption ; trees and plants are found buried 

 in them. The surturbrand of Iceland is of this origin. If the 

 sea washes the foot of a volcano, these, and other of its products, 

 will be mingled with marine deposits, and often carried by cur- 

 rents to a distance. This was the origin of the. stratified tufas 

 of Campania, and the environs of Rome. In Hungary, pumice 

 conglomerates alternate with tertiary limestone, as basaltic pe- 

 perinos do in Auvergne, the Vicentine, and the Val Demona in 

 Sicily. 



The craters of volcanic mountains are subject to a series of 

 changes, by which they are alternately filled up and emptied 

 again. The large crater, left by any paroxysmal eruption, is 

 gradually filled by the accumulating products of minor eruptions, 

 until it is replaced by a convexity of summit. This form is, as 

 we have seen, the most favourabe to the permanence of the 

 eruptive process ; but at the same time, the quantity of matter 

 accumulated above the focus, and, therefore, the obstruction to 

 the escape of the caloric in as quick a ratio as it is received 

 ihere from below, is at its maximum; consequently, the proba- 

 bility of the recurrence of a paroxysmal eruption, from this 



Vol. XIIL— No. 1. 17 



