GENERAL MEETING. of. 
of heat are the cause of voleanism. This is an arbitrary postulate, 
which, by its own terms, precludes discussion. Nevertheless it is 
the one which I believe agrees best, and perhaps perfectly, with ob- 
served facts. It undoubtedly sweeps away the difficulties which 
encumber all other hypotheses, but unfortunately it is an appeal to 
mystery, and therefore substitutes a single difficulty as great as, if 
not greater, than all the other difficulties put together. 
There is a fifth hypothesis, which takes account of the fact that 
many bodies which are solid under great pressure are immediately 
liquefied when the pressure is removed, heat being neither lost nor 
gained. The removal of pressure by denudation of the surface 
above the seat of lavas may thus determine volcanic action. The 
reply to this is that voleanoes do not always, nor even generally, 
occur where such denudation and consequent relief of pressure, are 
in progress. The true law of the distribution of volcanoes appears 
to be the one given by the late Charles Darwin, viz., that they occur 
in areas which are undergoing elevation. 
There are several broad facts, or categories of facts, which a true 
theory of the volcano must cover, and which will be recited 
briefly. 
1. Lavas, in their subterranean seat, could not possibly have been 
in a highly elastic explosive condition from the earliest epochs of 
the earth’s evolution, and only waiting a convenient season to break 
forth. We have no alternative but to regard them as being inert 
and inexplosive in their primitive condition, and as having acquired 
explosive energy just before the epoch of eruption. To assume that 
they have always been in the condition they present while pouring 
forth, and that the opening of a fissure has been the accident which 
determined the eruption, is reasoning ina circle. It is the energy 
of the lavas which causes the fissure, and not the fissure which 
causes the lavas to extrude. The lavas extrude themselves by vir- 
tue of their acquired elastic force. The theory must explain how 
materials which antecedently were inert, passive, incapable of erup- 
tion, may become active, dynamical, eruptible. 
2. Another broad fact, closely related to the foregoing, is the in- 
termittent action of volcanoes. These vents do not discharge all 
their available products at once, but by repeated spasms of activity, 
separated by longer intervals of repose. If these fiery explosive 
liquids had lain so long in the earth, chock-full of energy and only 
awaiting the opening of a passage-way, how happens it that when 
