The Causes Of The Ascensive Force Of The Lava Column are sought by various writers in several different agencies. Some find an all-sufficient cause in the steam pressure, while others maintain that some other force must be at work and find this partly in the weight of overlying masses, especially in the case of sinking blocks, and partly in the unequal contraction of the earth, and consequent pressure upon the molten or plastic layer beneath. It has been calculated that a radial contraction of one millimetre "would suffice to supply matter for five hundred of the greatest known volcanic eruptions." (Prestwich.) That steam is an essential factor in this, as in other volcanic phenomena, appears to be well established. Steam and other gases and vapours, under great pressures and at high temperatures, have a remarkable penetrating power and, when suddenly released, will perforate metal or rock like a projectile. Even under a pressure of only a few hundred atmospheres, superheated steam corrodes and abrades like the sand-blast, as a substitute for which it is now frequently employed.

(5) The Intermittency Of Volcanoes

The Intermittency Of Volcanoes and their mode of distribution add to the difficulty of the whole subject, but any complete theory must explain them. The views which bring volcanic action into relation with the mechanical changes in the crust are those which seem most consonant with the known facts of the past and present distribution of the vents. .

Here, for lack of space, we must leave the subject. Enough has been said to show how far we still are from understanding the mystery of volcanoes.