95 
M. Necker, of Geneva, offered an addition to the causes of con- 
vulsions of the earth, which are contemplated by our Geological 
Dynamics, in a paper in which he ascribed the earthquakes which 
took place in the southern provinces of Spain, in 1829, to the falling in 
of strata, the subjacent gypseous and saliferous masses being washed 
out by subterraneous currents. Without denying all influence to 
such a cause, we may observe that it does not appear likely that 
there would be thus produced, simultaneously, any greater effects 
than those which are known to have occurred from the falling in 
of unsupported mines; and these have never approached in their 
scale to any except the smallest earthquakes. 
While geologists are thus looking in all directions for causes which 
may produce the phenomena which they study, it is natural that the 
powerful, but as yet mysterious influences of electricity should draw 
their attention. Mr. Robert Were Fox has endeavoured to show, 
that by voltaic agency, a laminated structure, and deposits of metal 
in cracks, resembling metallic veins, may be produced in masses of: 
clay. The experiments are of an interesting kind, and it can hardly 
be doubted that voltaic agency had some influence in such cases 
as those described by Mr. Fox; although Mr. Henwood and Mr. Stur- 
geon have failed in attempting to reproduce his results, and although 
results much resembling these occur in cases where no electrical ac- 
tion is suspected. But we may remark that the conditions under 
which such voltaic effects are produced have not yet been attempted 
to be defined with any accuracy ; and that till this is done, the reality 
of such agency can neither be verified nor applied to geological 
speculations. — 
_ A’reflection which naturally offers itself upon this review of our 
recent career, is this :—that different portions of the science of geo- 
logy advance with very different rapidity. Descriptive Geology is con- 
stantly and actively progressive: facts are accumulated by observers 
in every land; and though facts are, in truth, of no value, at least 
for any purpose of science, except so far as they are reduced to some 
classification, yet on the other hand, sound classifications are perpe- 
tually, almost necessarily, suggested, when observation is vigilant 
and persevering. Even if we at first express our facts in terms of a 
