EXPULSION OP LAVA.

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CHAPTER VI.

OUTFLOW AND DISPOSITION OF LAVA.

§ 1. The emission of lava is usually as characteristic a featureof volcanic eruptions as those aeriform explosions which havebeen described above. Some volcanos, it is true, send forthvast quantities unaccompanied by any corresponding explo-sions,—the expanding steam and gases issuing in close inter-mixture with the intumescent matter, instead of rising throughit and escaping in those vast bubbles already described asusually bursting from its surface so soon as a communicationis obtained with the atmosphere. This difference is, probably,in great part due to the different specific gravities of the lavasin the two cases.

It generally happens, however, that a quantity of lava issimultaneously expelled in a more or less fluid state from thesame orifice as the fragmentary ejections. Should the conehave been formed first, this matter may rise so as to fill thecrater up to the lowest lip of its ridge, and thence flow downthe outer slope, leaving there a crust or bed of scoriform orsolid rock; but more frequently its enormous weight andpressure break down the entire side of the cone, composed,as this always is, of loose incoherent matters, whence it escapesin a flood or * current’ towards some lower levels. Many of thepoints of eruption on the flanks of Etna are marked by sucha cone of scorise broken down on one side by an outburst oflava. Breached cones form, indeed, one of the most ordinaryfeatures of every volcanic region. In Iceland and Teneriffethey are numerous. Several occur in the extinct volcanicdistrict of Sardinia , according to the report of General La

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