742 R. MALLET ON AN HITHERTO UNNOTICED CIRCUMSTANCE 



the outburst and it would have been highly important to know 

 whether in this, as in many other observed cases, not far distant 

 from active volcanic vents spots or portions of the ground at the 

 surface had become more or less highly heated before any visible 

 mechanical disturbance had taken place. In all volcanic districts 

 numerous spots are to be found where the ground is highly heated 

 by the emission of small divided streams of steam (probably super- 

 heated), and of heated gases : but this is not a case in point ; for 

 here the heat is evected and brought to the surface by these elastic 

 fluids, of which well-marked examples are found at II Stufi, in the 

 Island of Lipari, and in the neighbourhood of Pozzuoli. In the 

 island of Yulcano, however, (one of the Lipari group,) I observed 

 well-marked instances where the ground, consisting of volcanic 

 pebbles and sand, was too hot to stand upon ; and in one case, a few 

 yards outside the sea-margin the sea-bottom of volcanic sand was 

 too hot to be borne by the naked foot, although the sea-water 

 covering the bottom was nearly a yard in depth ; yet here not a 

 bubble of gas (steam might have been condensed) was seen to be 

 emitted or to rise from the water. Many other instances of heated 

 soil without emission of steam or gases are to be found scattered 

 through the pages of observant travellers ; and I need not adduce in- 

 stances from these. We may be justified in presuming that this sort 

 of local thinning and softening of certain spots may be one of the 

 ways (however incapable as yet of being explained fully in all its 

 details) in which new volcanic vents are opened. 



However the penetration of the surface may take place, the 

 material which is ejected from the opened orifice is deposited and 

 heaped up around, the ring-formed mound being continually increased 

 in height and in diameter by additions of material showered upon 

 it, or added to it in a liquid state by overflow or injection. If the 

 origin d penetration be through a level plane surface, whether of 

 discontinuous or of coherent rock-material, the first deposits must 

 conform to the contour of the plane on which they rest. But as 

 deposition proceeds and each layer of matter is brought to rest upon 

 the uppermost of those previously deposited, so, could we dissect 

 and separate the successive layers of deposit, we should find that 

 the inclination of the layers, at least upon the exterior side of the 

 ring-formed mound, continually increased, up to a certain limit of 

 slope or angle of repose, dependent upon the nature of the material 

 itself — after which the angle of slope of succeeding deposits does not 

 increase, but the slope is more or less disturbed and modified by 

 great slippages of the already deposited material taking place at one 

 or at many points round the circumference. Such slipj>ages also 

 occur, and still more frequently, from the internal slope ; and these 

 falling back into the crater-orifice are again ejected ; but the interior 

 of the mound or cavity of the crater, being in fact constantly exposed 

 to a high temperature, and to the baking and chemical actions due 

 to contact with the torrent of heated gases, vapours, and solid 

 ejecta rushing past, becomes like that of the vast Hue of a furnace, 

 indurated; so that the angle of the interior of the crater is not the 



