ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
217 
constitution of the lava, and the higher or lower temperature of the focus ; and we can 
further see that the constitution of the lava has some relation to the successive lithologic 
formations in which the focus and vent are situated or through which they pass. 
Siliceous crystalline rock and aluminous rocks alone, pulverized and fused, produce, 
for example, highly infusible lavas ; siliceous and calcareous, still more those with 
certain portions of aluminous or ferruginous rock, much more fusible ones. The old 
hut valuable experiments of Kiewan are worthy of being still consulted on these points 
(Kiewan’s ‘ Mineralogy ’), the fusibility in all cases also being largely influenced by the 
alkaline contents of the water, sea or fresh, that finds its way to the focus. 
214. Now all these conditions are accountable for on the supposition of local and often 
more or less distinct foci of heated and pulverized rocks, differing in composition at 
different depths of the heated column. 
We have also an adequate cause for the great differences of temperature at different 
and even closely adjacent vents, in that the heat at the focus is not derived from any 
invariable source at a nearly constant and uniform temperature, as in Hopkins’s notion, 
hut is directly proportionate to the local tangential pressure which produces the crushing 
and the resistance thereto, and may vary to any extent at different points or at the same 
point at different times. 
215. In this, too, we find an adequate and easy explanation of the absolutely non- 
periodic activity of volcanoes and their occasional sudden and violently paroxysmal 
action, as well as for their long periods of repose, and for the absolute extinction of 
some and the breaking out of new ones at points where none previously existed. 
216. The secular cooling of the globe is always going on, though in a very slowly 
descending ratio. Contraction is therefore constantly providing a store of energy to be 
expended in crushing parts of the crust, and through that providing for the volcanic 
heat. But the crushing itself does not take place with uniformity, it necessarily acts 
per salturn after accumulated pressure has reached the necessary amount at a given 
point, where some of the pressed mass, unequally pressed as we must assume it, gives 
way, and is succeeded perhaps by a time of repose or by the transfer of the crushing 
action elsewhere to some weaker point. 
217. Hence, though the magazine of volcanic energy is being constantly and steadily 
replenished by secular cooling, the effects are intermittent, and just provide from year 
to year the amount of force which is consumed in vulcanicity. 
It is one of the many cases in nature in which the uniform development of a force 
results in variable and intermittent action as the effect of the force ; it is steadily pro- 
duced and accumulated, but unequally or paroxysmally expended. That such slowly 
accumulating pressure on local points of rigid solids does produce their giving way by- 
crushing paroxysmally may be illustrated to the senses by pressing slowly and steadily 
a lump of sugar held by the fingers against a table ; some of the points or surfaces in 
contact crush to powder, there is a momentary repose ; we continue the pressure, or 
slowly or slightly increase it, more crushing, and another repose follows. 
