348 Analysis of Scientific Books and Memoirs. 



and basalt not having been confined, as of right, to a mineralogical mean- 

 ing. 



Having thus far traced the laws which determine the disposition of the 

 substances produced by a single volcanic eruption, whether in a fragmenta- 

 ry form, or as more or less liquid lava, the author proceeds to examine the 

 circumstances that result from the accumulation of such products, by re- 

 peated eruptions from the same vent. The simple cone, by this process, 

 becomes enlarged into a volcanic mountain, composed of hardened lava- 

 streams, (each of which acts as a solid rib or buttress to the hill,) and in- 

 tervening beds of conglomerate. The sides of this hill are frequently, 

 during eruptions, split by the pressure of the column of lava, within the 

 central aperture or chimney of the volcano. The lava then flows out 

 through orifices, formed successively at different levels, one below the 

 other ; examples of such occurrences are given from the phenomena of 

 iEtna, Vesuvius, Iceland, &c. It is a general fact, that, in the eruptions 

 of volcanic mountains, or habitual volcanos, the elastic fluids are chiefly 

 discharged from the central crater, but the lava is emitted from apertures 

 in the side, or at the base, of the mountain. Minor and local earthquakes 

 are occasioned by these rendings of the frame-work of the mountain, which 

 is even sometimes split in two. The consolidation of the lava that occu- 

 pies these fissures, produces numerous vertical dikes, which, cutting 

 across its 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 increases, 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 thown 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 con- 

 siderable 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 conglomerates of an allu- 

 vial 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 mud and water, are called by the inhabitants of 

 Vesuvius, lave d'acqua. 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 alternating with the 

 currents of lava which have flowed farthest from the centre of eruption ;«— 

 trees and plants are found buried in them. The suriurbrand of Iceland 

 is of this origin. If the sea washes the foot of a volcano, these, aud other 

 of its products, will be mingled with marine deposits, and often carried by 



