216 
ME. ROBEET MALLET ON VOLCANIC ENERGY. 
from even nearly the same depth ; but the depth must be nearly the same for all if the 
foci be in a spherical couche of liquid lakes left by refrigeration of the rest of the globe, 
as imagined by Hopkins. 
209. These difficulties disappear on our theory. The focus of heat may be at any 
depth, because crushing of the solid crust may occur at any depth, dependent on thickness 
of crust, couclie of maximum tangential pressure or of greatest crushing resistance, &c. ; 
and in general the tendency must be for the crushing simply to occur at no great depths 
from the surface. 
210. The crushing is local, both as to surface and depth ; where it occurs, being in the 
weakest parts of the crust, the fissures for admission of water are the most likely to be 
present. 
211. The result of the crushing is to produce irregular masses, on the whole tending to 
verticality, of pulverized rock, heated more or less highly, that may extend to any depth 
within the solid crust ; but it is only to such depth as water can percolate or infiltrate by 
capillarity that the deepest focus of volcanic activity can be found. 
Below that the crushed and heated rock may exist, but it remains quiescent unless 
water reaches it or gases be evolved by chemical action increased by the heat. When 
water does reach such a heated mass of crumbled rock, it readily finds its way through 
the whole mass, which absorbs it as red-hot sand does water poured into it. Steam is 
produced if the temperature due to crushing be sufficient to raise both the crushed rock 
and the water (under the pressure of its superincumbent column and the resistances of 
the water-ducts) to the fusing-point, boursoifie lava results, and at sufficient elastic 
steam pressure is ejected, perhaps enlarging its own vent of issue by some preexistent 
fissure, by fusion as it rises, and subsequently by abrasion. 
212. The researches of later years, and especially of Jam in and Daubree, have shown 
that infiltrating water through the capillary pores of permeable rock may continue to 
pass through such against a heavy steam pressure ; so that whilst the water continues to 
enter thus a heated cavity full of hot crushed rock, urged by an insistent head and by 
capillarity in the rock, no steam can escape back through the porous rock preoccupied 
by the water, just as a porous filtering stone under a head of water would continue to 
pass water into a fire, though the latter contained steam or gases under great pressure 
which could not pass out through the stone. We have thus all the conditions in our 
focus needed for the production of such variations of lava as we actually witness. 
Analyses have shown that while there is a great general similarity of constitution in 
lavas all over the globe, they yet do differ enough in constitution considerably to affect 
their degree of fusibility. We are also able to observe that some volcanic vents produce 
more fusible lavas than others, that some volcanoes produce little lava and much heated 
and pulverized material, and some nothing but the latter, no lava at all. 
213. We can also see (in a good many cases at least) that the fusibility of the lava, and 
the proportion of its supply to that of unfused heated matter, dust, and lapilli, is refer- 
able to two (coexistent or not) elements of cause — the more or less fusible chemical 
