354 J. W. Judd — On Volcanos. 



through the removal by denudation of the overlying volcanic 

 structures, a cause for the varying modes of action at different points 

 of the same volcanic district may readily suggest itself. 



The great fissures filled with consolidated materials, which pene- 

 trate older rocks in volcanic areas that have suffered great denudation, 

 affect two very distinct modes of arrangement. They are either 

 cracks which traverse the strata vertically, or fissures which have 

 been formed through the yielding of the planes of least resistance 

 among the strata themselves. The former, filled with consolidated 

 lava, become dykes ; the latter, intrusive sheets. 



That the fissures of both classes sometimes reached the surface, and 

 that, in such cases, they gave origin to volcanic outbursts, we have 

 very unmistakeable evidence. But it is also clear that the action 

 which would take place at the surface in the case of the two kinds of 

 fissures would necessarily be very different. In the case of a vertical 

 fissure, the smallest communication with the surface wouldlead to alocal 

 disengagement of vapour, and this relieving the pressure on the mass 

 below, continually fresh supplies of steam would be liberated, carry- 

 ing up fragments of the liquefied rock in which it was imprisoned as 

 scorise or pumice, or forcing it out in streams as lava. Thus would 

 naturally be built up, according to circumstances, a cone of cinders, 

 a composite cone of cinders and lava, or a solid cone ("mamelon"), 

 wholly formed by the welling out of the latter material. But in the 

 case of a horizontal fissure, the result would probably be very differ- 

 ent. Here the mass of lava, which, as we know, may be forced 

 for many miles away from the volcanic centre, would have its im- 

 prisoned water retained by the superincumbent rocks till it reached a 

 point at which, either from a decrease in the thickness or a diminution 

 in the capacity for withstanding expansive force of the superincumbent 

 rock, it began to be disengaged. Then an accumulation of vapour of 

 the highest tension would begin to take place, and by its accumulated 

 force, the repressive power of the overlying rocks being at last com- 

 pletely overcome, the latter, throughout a wide area, would be 

 shattered to fragments and dissipated in one short, sudden, and 

 violent outburst. But the mass of lava to which this outburst was 

 due, having beneath it no further reservoir from which steam could 

 be disengaged and rise to the surface, the first violent outburst 

 would not be succeeded, as in the case of vertical fissures, by a 

 series of similar explosions. 



By the liberation of vapour in vertical and horizontal fissures re- 

 spectively, then, it seems possible to account for the formation in the 

 same district, as in the Campi Phlegrasi, of the two very distinct 

 kinds of volcanic vents, or for the appearance of either class almost 

 alone, as in the Eifel and the Auvergne. 



But though this explanation may suffice to account for the 

 production of those smaller vents which occur in such areas as we 

 have referred to, yet it is evident that the formation of enormous 

 craters like those of Bracciano and Bolsena is a problem of a different 

 and perhaps far more difficult character. 



If, for example, we were to conceive of an eruption of so violent a 



