546 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 2, 



and of a bulk and dimensions determined by the comparative fluidity 

 of the expelled lavas and the comparative abundance and character 

 of the fragmentary ejections. When, as often is the case, owing to 

 the obstruction of the central vent, eruptions, whether of lava, or 

 of scoriae alone, or of both together, have occurred repeatedly on 

 the lower flanks or near the base of the volcanic mountain, its form 

 will have been proportionately varied, by the greater bulk added to 

 its lower as compared with its higher slopes. Often some great 

 paroxysmal eruption will have broken a huge crater through the 

 core of the mountain thus formed, blowing off its summit and fairly 

 emptying its bowels, leaving a truncated cone, perhaps only a " basal 

 wreck," like the roots of a hollow tree-stump, wholly or partly en- 

 circling the cavity, which subsequent eruptions from its interior may 

 not have been able as yet to fill up. Sometimes two or three of such 

 encircling cones and craters will have been successively formed, one 

 within the other, round a common centre of eruption. Occasionally 

 the habitual vent will have been so strongly sealed up by the con- 

 solidation of the lava contained in it, or the accumulation of ejected 

 matters above it, as to force the subsequent eruptions to shift to a 

 new point of the same, or some newly broken, fissure ; and the 

 axial centre of the mountain will thus have changed its place, giving 

 rise to an elliptical figure and other corresponding irregularities in 

 the external form and also in the internal structure of the mass. 

 The strata of the new cone will rest in such cases unconformably 

 on those of the old, as we see is the fact in Etna. This must also 

 be the case in the smaller cones of scoriae, the product of a single 

 eruption, when the position of the vent has slightly varied, as hap- 

 pened in the case of the Puy Pariou in Auvergne. (See fig. 25.) 



Fig. 25. — Ideal Section of the double cone of Pariou, Monts de Dome. 





What I maintain is, that, making allowance for these several 

 varying circumstances, all of which are within the range of admitted 

 and indisputable experience in the instances of eruptions actually 

 witnessed, there is nothing in the form, structure, or mineral cha- 

 racter of any volcanic mountains or formations yet observed, which 

 cannot be explained by the simple, intelligible, and consistent laws 

 of volcanic action briefly described above, and that consequently 

 there is no need for the supposition of a sudden circular upheaval of 

 previously horizontal beds of volcanic matter to explain the mode of 

 production of any one such mountain — far less of all, or nearly all, to 

 which the upheavalists insist on applying their theory. 



Of course it is not intended to deny — on the contrary, it is an 



