344 Prof. J. Milne — On the Form of Volcanos. 



of Oshima, of whicli, in 1877, I was a close spectator, commenced by 

 the throwing up of ashes from a fissure which had for several years 

 previously only given vent to steam. At the time of my visit a 

 quantity of these had accumulated to form a heap of a certain 

 height. The form of this heap was to all appearances similar to that 

 of the larger mountains. For this heap to increase in height, so long as 

 the plane on which it rested was approximately level, it was necessary 

 that it should also increase the diameter of its base. Now, if the 

 sides of a volcanic mountain have slopes which are always loga- 

 rithmic curves, and if we assume that the area of the top is always 

 some fraction of the height, the relative rates at which the height 

 and the diameter of the base increase can be easily calculated, — and 

 if we have given the height of a mountain, we shall, under the 

 given circumstances, be able to determine the diameter of its base. 



The eruption at Oshima has now ceased, and the materials which 

 have been ejected may be regarded as the nucleus of a small 

 mountain. By such causes as the action of its own weight, 

 infiltration, etc., this mass is now gradually becoming consolidated. 

 As this embryonic mountain is of the best possible shape to resist 

 the effects of denudation and weather generally, from wearing much 

 from its sides, this consolidation will leave it in a form not very 

 much imlike that in which it was first raised. Should a second 

 eruption take place from or near the old crater, it will have to pile 

 material on this hardened nucleus, which, being stronger than a mass 

 of loose ashes, will act like a box beneath a heap of loose sand, 

 rendering the mountain generally steeper. 



We have evidently here in certain cases a rough test as to whether 

 any mountain has been produced by one or more large eruptions, 

 and at the same time an insight into the probable method by which 

 many volcanic mountains have been raised. 



Where there have been many overflows of lava, these may have 

 consolidated like ribs or buttresses on the sides, and have tended to 

 make the nucleus still stronger, and, therefore, prepared after the 

 next eruption to support material at a still steeper angle. 



As the mountain increases in height, its weight, as pointed out by 

 Mr. Mallet, probably produces a depression in the base on which it 

 rests, this being greatest under the apex of the mountain, where the 

 pressure is greatest. The ultimate result of all this will be that the 

 mountain will tend to rest in a saucer- shaped hollow, and to have its 

 lower beds dipping radially towards a centre. If this action should 

 have taken place in any mountain, it might at first sight be con- 

 sidered as influencing the external form of the mountain, and 

 consequently any deductions which have been drawn from such a 

 form. However, we must remember that this sinking will, as com- 

 pared with the mass of the mountain, be small, and, also, it will be 

 gradual, so that, without appealing to calculations, I think it will be 

 admitted that the accumulation of material will more than counteract 

 such an action. Even should there be a sudden collapse beneath a 

 mountain tending to produce any inequalities on its external surface, 

 these inequalities will represent material in an unstable position, 



