62 RICHTIIOFEN — NATURAL SYSTEM 



features are generally least conspicuous. This basic compound is not only more fusi- 

 ble than the more silicious rocks; but, it appears that the admixture of superheated 

 water will increase its fluidity more than that of the other compounds, while those 

 unknown influences which cause the mass to solidify in the particular form of basalt 

 must be still more potent in increasing its fluidity, since lavas consisting of dolerite, 

 leucitophyre, and trachydolerite, which are cpiite or nearly identical with the former 

 in chemical composition, are never so liquid as those of basalt, and are not unfrequently 

 quite viscous. These rocks, together with all those of a more silicious composition, ex- 

 hibit more distinctly the differences in origin, and this is probably due, in a great meas- 

 ure, to the peculiarity just mentioned. The great fluidity of basalt, which is also made 

 evident by its frequent occurrence in very thin and yet very extended dykes, causes 

 it, even when confined in the narrow space of a volcanic vent, to let the vapors of 

 water escape, in quiet ebullition, in its orifice, as Dana has so beautifully illustrated in 

 his description of Kilauea, while, at intervals, it will break out and cover the sur- 

 rounding country with flat sheets of lava. The action connected with the ejection of 

 the same rock from larger fissures, in former time, appears to have been similar to 

 this. Yet, the numerous instances of the occurrence of basaltic cinder cones, and, on 

 the other hand, of large accumulations of solid basalt with no perceptible horizontal 

 structure, go to show thai also in the case of this rock the modes of occurrence may 

 be different when the}' are the result of different modes of ejection. Other instances 

 of a similarity of the manner in which the matter has been deposited, when due to 

 either mode of ejection, are frequent on the flanks and at the ends of the andesitic 

 ranges of Hungary, where currents of andesite as well as of rhyolite have been emitted 

 through fissures in andesite, at little elevation above the foot of those ranges. They 

 appear to be due to processes intermediate in kind between both modes of ejection. 

 We may, finally, mention those cases where massive eruptions were sub-aqueous, and 

 layers of fine-grained tufa formed, alternating with coarser conglomerates, between 

 which may lie intercalated solid layers of the same kind of rock of the fragments of 

 which those are composed. The similarity of this kind of depositions with the sedi- 

 ments of submarine volcanoes is often very great. 



The principal point of difference between massive eruptions and volcanic action 

 appears to be the depth of their source under the surface, and all the minor differences 

 are probably dependent upon that. The region from which the former have derived 

 their material, is, as we tried to prove, at a great depth beneath the deepest sedimen- 

 tary rocks. The seat of volcanic action appears to vary within wide limits in regard to 

 its distance from the surface, but to be, on an average, at much less depth than that of 

 the massive eruptions ; though there are circumstances which render ir probable that 

 it is in all cases beneath the shell composed of sediments. Evidence has been gathered 

 by Prevost, Dana, Scrope, Hopkins, and others, in favor of the assumption that vol- 

 canoes are not connected with the molten interior of the globe, and arc therefore not 

 to lie considered as safety-valves. The comparatively little distance of the seat of vol- 

 canoes beneath the surface is rendered particularly evident by the small area of the 

 earthquakes attending their activity, when compared with the wide extent of others 

 which must be dependent on some deep-seated action, hut have no recognizable con- 

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