ME. EOEEET MALLET ON VOLCANIC ENEEGY. 
151 
An unlimited supply of liquid lava was thus provided ; the question was, what brought 
it up through this thin crust, and ejected it and other matters at the surface. By some, 
amongst whom Signor Belli may be distinguished for the ability of his writings, the 
crust was assumed so thin as to merely float upon the liquid globe beneath; and a 
mechanism being conceived for fracturing the crust into separate fragments, and their 
density being rather greater than that of the supporting fluid, these sunk more or less 
in the latter and forced up the liquid lava in the spaces between. 
13. This, which may be called the hydrostatic theory, has had several modifications. 
Another school of geologists, and the more numerous one, assumes that infiltrated water 
reaches the liquid and incandescent nucleus at certain points, and that by the elastic 
force of the stream formed the lava and other ejecta are forced up ; and this, too, has had 
several modifications. 
These latter views are no doubt true so far as the general notion of ejection by steam 
pressure is concerned ; but insuperable difficulties appear to arise against the origin of 
the incandescent matter coming from one great reservoir, common, therefore, to all the 
volcanoes on the earth. 
14. There is no evidence of universal connexion with such a common source of liquid 
rock beneath. 
On the contrary, volcanic vents even closely adjacent show no proofs of direct inter- 
communication. 
Their efforts are not synchronous; their paroxysms are isolated and subject to no 
recognizable periodicity ; their ejecta, solid, liquid, or gaseous, though showing a great 
general similarity in all parts of the world, are far from being identical in chemical con- 
stitution or in temperature ; these vary at different times, and show signs of secular 
change in geologic time. 
The liquid or solid ejecta show no such uniformity at all volcanic vents as should 
arise from their coming all ready fused from one universal reservoir, the contents of 
which, at the same depth, it has been supposed there is no ground for assuming to differ 
in composition ; while they do show very distinct indications of having some relation 
to the rocks directly through which the vents pierce, or over which they are posited. 
None of these difficulties, to which others of a not unimportant character might be 
added, have ever been explained away. 
15. A further difficulty was placed in the way when the investigations taken from astro- 
nomical considerations were supposed to prove that the earth’s crust, instead of being 
very thin , must be very thick. It was not inconceivable that the liquid rock of the 
nucleus might in some way reach the surface through ducts or fissures in a shell of 20 
or GO miles thick ; but it was at least more difficult to conceive, if not quite inconceivable, 
by what force and how it should be propelled through ducts of 800 or 1000 miles or even 
more in depth. It is quite a separate question what degree of weight should be attached 
to the celebrated paper by the late Mr. W. Hopkins, F.R.S., on Precession and Nutation, 
as proving a thickness of solid crust exceeding 800 miles or so. 
The difficulties to his reasoning, arising from his neglect of viscosity and friction- 
