7 5 
dynamical causes of this class of phenomena, I adopt, as I 
have already intimated, the theory advocated by Herschel in the 
before-mentioned Lecture. After giving details of many extra- 
ordinary effects attributable to earthquakes and volcanoes, he 
goes on to say, “The origin of such an enormous power thus 
occasionally exerting itself will no doubt seem very marvellous 
little short, indeed, of miraculous intervention ; but the 
mystery, after all, is not quite so great as at first it seems. We 
are permitted to look a little way into these great secrets of 
Nature; not far enough, indeed, to clear up every difficulty, but 
quite enough to penetrate us with admiration of "that wonderful 
system of counterbalances and compensations ; that adjustment 
of causes and consequences, by which, throughout all nature, 
evils are made to work their own cure ; life to spring out of 
death ; and renovation to tread in the steps and efface the 
vestiges of decay” (10). He then asserts categorically that 
“ the key to the whole matter is to be found in the central heat 
of the earth ” (11) ; and before proceeding to indicate liow this 
key unlocks the mystery, he requires nothing more than that 
there should be granted him “a sea of liquid fire, on which we 
are all floating, land and sea ; for the bottom of the sea will not 
come nearly down to the lava-level, the sea being probably no- 
where more than five or six miles deep, which is far enough 
above that level to keep its bed from becoming red-hot” (16). 
It will be seen, on referring to the preliminary argument at the 
beginning of Division II., that the above postulate may reason- 
ably be granted, if, as is there maintained, the quality of rigidity 
is destroyed in a very large proportion of the earth’s interior 
mass, both by pressure and by heat, so that the dynamical 
properties of the mass become the same as those of a perfect 
liquid. In that case the transfer of ever so small a quantity of 
material from one position to another on the eai'th’s surface, will 
tend to disturb the equilibrium of the floating mass. This cause 
of disturbance will not, however, immediately take effect, because 
the viscosity and rigidity of the earth’s crust will act conjointly 
as an opposing force; but whatever he the amount of resistance 
this obstacle is capable of, it has a definite limit, and must, 
therefore, eventually yield to the constantly increasing disturbing 
force due to the accumulation of transported matter, both from 
the detritus of mountains and cliffs, and from the mud and 
gravel and sand conveyed by rivers. In short, the mechanical 
operation and its effects may be very appropriately described in 
the words of Herschel, contained in the following passage : — 
“ It is impossible but that this increase of pressure in some 
places and relief in others must be very unequal in their 
