540 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Peb. 2, 



abundance and excess of matter there, solid, no less than fluid and 

 gaseous, that struggles to find a vent, and having obtained one, 

 the mountain continues to discharge the redundant contents of its 

 interior until the plethora is reduced, and the forces of repression, 

 consisting in the weight and tenacity of the overlying mass (together 

 with the weight of the atmosphere, which should not be overlooked), 

 recover their ascendency and stop any further evacuation. This is 

 a state of things the very reverse of that imagined by the up- 

 heavalists, namely a great internal void capable of swallowing up at 

 one shock the whole upper crust of the mountain. 



No doubt, whenever a deep crater has been formed within the axis 

 of a mountain by some paroxysmal eruption, surrounded by vertical 

 cliffs or fractured rock (the hollowed shell or lateral crust, as it were, 

 of the eviscerated cone), the occurrence of a violent earthquake-shock 

 may shake down great part of this fragile enclosure and cause it to 

 be precipitated into the depths of the crater, by which fall the cone 

 will lose a proportion of its height, and the crater itself be widened 

 and partially filled. Such an event seems to have occurred on Vesu- 

 vius in 1761, according to Hamilton ; but such an occurrence in no 

 degree corresponds to the engulfment of an upheaved dome imagined 

 by the upheavalists. 



It is also not improbable that some of the small ^i^- or lake-craters, 

 such as the Cisterna on the Piano del Lago near the summit of Etna, 

 the Maare of the Eifel, and the Lacs Pavin, Thavana, and others in 

 Central France, on whose borders are found but a very slight sprink- 

 ling of scoriae, volcanic bombs, and other fragmentary ejecta, may 

 owe their origin to violent explosions of very short continuance, pro- 

 ceeding from a local accumulation of vapour, upon the discharge of 

 which the bulk of the overljing beds may have fallen back into the 

 cavity left by the explosions. Many of these small craters were 

 evidently broken through pre-existing rocks — granite, clay-slate, 

 basalt, or other, the fragments of which are scattered aroimd ; but 

 in no case that I am aware of is there any sign of the tilting, eleva- 

 tion, or even the derangement of the bedding of these rocks imme- 

 diately around the craters — certainly no appearance of their conoidal 

 or dome-like upheaval. What scoriae or other fragmentary dejec- 

 tions occur about such craters are arranged in quaquaversal taluses, 

 as usual, but quite unconformably to the old rocks through which the 

 eruption broke out, which clearly retain their former position, what- 

 ever it was, around the cavity forcibly drilled through them. Nor, 

 indeed, could this class of craters lend any support to the views of 

 the upheavalists, since they admit them to be of eruptive origin*. 



Another kind of pit- crater is that which occurs in several instances 

 in the Sandwich Islands, and of which Kilauea is the best example. 

 Where a cone has been formed by the repeated overflow of a highly 

 liquid lava boiHng up from a central vent, should its flanks at any 

 time give way to the hydrostatic pressure of the internal column, 

 or other cause, and a fissure be formed through which the interior 

 is, as it were, tapped of the greater paii; of its contents, a circular 

 * Humboldt, Kosmos, iv. p. 230. 



