[2] (P. 94). It can scarcely be supposed that these small eruptive-looking belchings forth from the lava stream, _en route_, are truly of an eruptive nature at all, _i.e._, in any way connected with forces seated deeply beneath the bed of the lava stream, or in any way connected with the volcanic ducts of the cone or beneath it. They are most probably merely the bursting upwards of large bubbles; that is, of cavities formed in the mass of the more or less liquid lava by intestine movements, as its mass winds and rolls along, and by the aggregation of smaller cavities--all being filled with steam and gases--together with dust and volatile products which are ejected when the cavity opens up, and its contents escape at the upper surface of the lava stream in virtue of the continuation of the twistings and convolutions due to the stream motion itself, and to the unbalanced hydrostatic pressures acting upon the parietes of the bubble. Very large single bubbles of like character rise in the fluid lava within craters in vigorous action, and often so regularly that their recurrence causes a sort of rhythmical rush and roar in the column of steam, etc., issuing above the mouth. This was evident in the discharges issuing in 1857 from the highly instructive minor _bocca_, then existing, examined by me, and referred to ("Report, Naples, Earthquakes," etc. Vol. II., pp. 313, 314), as presenting at the time great facilities for determining pyrometrically the temperature of the lava within, and of the dry superheated steam issuing with a rhythmic roar from it. M. Le Coq ("Époques Géologiques d'Auvergne," Tome IV.) has recorded some examples of the formation and opening-out of large bubble-like cavities in lava already ejected. Perhaps that able and laborious vulcanologist, whose death a few months ago science still deplores, attributes too much importance as well as magnitude to them, when attributing the formation of what he has denominated "craters of explosion," to the mechanism of the rise and bursting of such bubbles upon a gigantic scale. Such blowings forth, sudden or prolonged, from particular spots of lava streams, _en route_, undoubtedly may also have their origin in damp places, or water or air-filled cavities in or beneath the bed over which the lava rolls, which, getting gradually heated, generate steam, or air or gases under tension by expansion, etc., which thus at length blow through the liquid or pasty lava flowing above, and which in bursting through delivers much dust also, and so simulates a little eruptive crater. Examples of this, upon a great and convincing scale, can be pointed to in the Val di Calanna and elsewhere on Etna.