152 
ME. EOBEET MALLET ON VOLCANIC ENERGY. 
hold between the solid crust and the liquid nucleus at the surfaces of contact, have been 
forcibly stated by M. Delaunay (and have certainly been underrated generally, and, 
indeed, have only been met by the mathematicians who have been the partisans of Mr. 
Hopkins’s views), by showing that if the physical difficulty of such viscosity &c. be admitted, 
other mathematical difficulties must arise if we cling to the method of Mr. Hopkins as 
capable at all of giving any answer to the question of thickness of the earth’s crust, 
which the writer believes it is not. That that thickness is not small, however, the writer 
believes to be the fact upon considerations wholly different from those of Mr. Hopkins ; 
and that view receives support from the investigation of Sir William Thomson as to 
the rigidity of the earth, at least with those who admit the sufficiency of the physical 
data upon which his mathematical reasoning there is founded. The effect, however, of 
this conjunction of the reasonings of the physical astronomer and of the geologist has 
been to raise a new difficulty for both. 
16. The geologist, chained down under SOOmiles or more of solid rock, the real thickness 
of which he is in no condition to disprove , cannot get his liquid lava sea beneath to the 
surface; and he has no other source for volcanic heat and ejecta to suggest. 
The mathematician has th e fact before him; volcanoes exist. He admits the difficulty 
of the geologist, and meets it by the most lame and gratuitous hypothesis of lakes or 
isolated masses of liquid fused rock existing at different points and at different depths 
(which depths, however, must, on the whole, be shallow within the solid crust of the 
earth), and assumes that from these the volcanic vents are supplied. 
17. Nothing can be morefeeble and unconvincing than the attempt made by Mr. Hopkins 
to give a rational explanation or support to this gratuitous and most improbable hypothesis, 
which, so far as the writer knows, that gentleman was the first to bring forward in his 
“ Researches in Physical Geology,” 2nd series, Phil. Trans, for 1842, Part II. : — “ We are 
necessarily led,” he says, “ therefore to the conclusion that the fluid matter of actual volca- 
noes exists in subterraneous reservoirs of limited extent, forming subterraneous lakes and 
not a subterraneous ocean ” (p. 51). He adds (same page), “ If we find that the hypothesis 
of the existence of these subterranean lakes, at no great depth beneath the surface, does 
enable us to account distinctly and by accurate investigations founded on mechanical 
principles for the phsenomena of elevation then we have all the proof of the 
truth of our hypothesis which the nature of the case will admit of.” 
That is to say, if we admit Mr. Hopkins’s physical notion of elevatory force, viz. that 
it consists of the pressure vertically upwards of a fluid against the superincumbent solid 
crust, we may also admit his lakes. If, however, as we shall presently see, Mr. Hopkins’s 
fundamental conception of the nature of elevatory forces is erroneous and untenable, 
then the lake-hypothesis must stand alone and upon its inherent improbabilities. At 
p. 52 the only attempt made to produce a rational origin for these supposed fiery lakes 
is thus given : — “ It would seem probable, I think, that their origin may be ascribed to 
the greater fusibility of the matter composing them ; and their continuance in a state 
of fluidity may, I conceive, be accounted for partly by the same and partly by another, 
which I will proceed to explain.” 
