62 RICHTHOFEN—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 quite 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 that also in the case of this rock the modes of occurrence may 
be different when they 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 be 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 voleanoes is often very great. 
The principal point of difference between massive eruptions and voleanic 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 it probable that 
it is in all cases beneath the shell composed of sediments. Hvidence 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 are therefore not 
to be 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, but have no recognizable con- 
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