584 Proceedings of the lioyal Society 
throughout ; or, at least, that its solid crust is of such thickness 
that the earth is to be regarded for all purposes as a solid body. 
Consequently the production of lava— that is to say, the liquefac- 
tion from heat of a small portion of the solid mass of the earth — is 
a strictly local occurrence ; this is sufficiently proved by the well- 
known fact, that the lava stands at different levels in different 
volcanic craters, even when they are in eruption at the same time, 
and very near each other : as in the case of Mauna-Loa and 
Kilanea, in Hawaii (Scrope on Yolcanoes, p. 262), which would be 
impossible if there were any free hydraulic communication between 
the different reservoirs of lava. The strata at great depth, not- 
withstanding their intense heat, are kept solid, as we have every 
reason to believe, by the pressure of the superincumbent strata, and 
their liquefaction in particular 'places , producing lava, is due , as I 
shall endeavour to prove, to local relief from pressure. 
Darwin has brought forward what appears to be nearly con- 
clusive evidence for believing that active volcanoes are to be found 
only in areas that are undergoing elevation. To quote his own 
words : — 
“ It may, I think, be considered as almost established, that 
volcanoes are often (not necessarily always) present in those areas 
where the subterranean motive power has lately forced, or is now 
forcing outwards the crust of the earth, but that they are invariably 
absent in those where the surface has lately subsided or is still 
subsiding.” — Darwin on Coral Islands, p. 142. 
It appears to be generally taken for granted that the expansive 
force of the earth’s internal heat is the motive power that produces 
elevation, and also volcanic action. In this theory, however, there 
is a difficulty which is not, I think, generally perceived. At the 
beginning of geological time, the earth must have been sufficiently 
cooled down to permit the formation over its surface of a cold solid 
stratum. The lower strata are not hotter now than they were 
then, and may perhaps be sensibly colder, as they are constantly, 
though very slowly, parting with heat. If, then, the temperature of 
the lower strata, millions of ages ago, was low enough to permit a 
cold solid crust to form above them, how can it now, when it has 
not risen, be high enough to raise up that crust in upheavals, and 
break through it in eruptions? It seems almost a contradiction to 
