428 
THE GEOLOGIST. 
would be sufficient to melt, under the ordinary pressure of the atmosphere, thl 
hardest rocks. Reasoning from these facts, it would appear that the mass ol 
the globe, at no great depth, must be in a fluid state. But this deductioJ 
requires to be modified by other considerations, namely, the influence m 
pressure on the fusing point, and the relative conductivity of the rocks whici 
form the earth's crust. To solve these questions a series of important experi- 
ments were instituted by Mr. Hopkins, in the prosecution of which Dr. Joule 
and myself took part ; and after a long and laborious investigation, it was ! 
found that the temperature of fluidity increased about 1° Fahrenheit for every 
5001bs. pressure, in the case of spermaceti, bees' wax, and other similar sub- 
stances. However, on extending these experiments to less compressible sub- 
stances, such as tin and barytes, a similar increase was not observed. But 
this series of experiments has been unavoidably interrupted ; nor is the series 
on the conductivity of rocks entirely finished. Until they have been completed 
by Mr. Hopkins, we can only make a partial use of them in forming an opinioi 
of the thickness of the earth's solid crust. Judging, however, alone from the 
greater conductivity of the igneous rocks, we may calculate that the thickness 
cannot possibly be less than nearly three times as great as that calculated iri 
the usual suppositions of the conductive power of the terrestrial mass at 
enormous depths being no greater than that of the superficial sedimentary 
beds. Other modes of investigation which Mr. Hopkins has brought to bear 
on this question appear to lead to the conclusion that the thickness of thf 
earth's crust is much greater even than that above stated. This would requir^ 
us to assume that a part of the heat in the crust is due to superficial andp 
external, rather than central causes. This does not bear directly against th^ 
doctrine of central heat, but shows that only a part of the increase of temperai 
ture observed in mines and deep wells is due to the outward flow of that heat.'* 
ADDEESS TO THE GEOLOGICAL SECTION. 
By Sir R. I. Murchison, D.C.L, LL.D., F.R.S. 
Although I have had the honour of presiding over the geologists of the British 
Association at several previous meetings since our first gathering at York, now 
thirty years ago, I have never been called upon to open the business of this 
section with an address ; this custom having been introduced since I last occu- 
pied the geological chair at Glasgow, in 1855. 
The addresses of my immediate predecessors, and the last anniversary dis- 
course of the President of the Geological Society of London, have embraced so 
much of the recent progress of our science in many branches, that it would be 
superfluous on my part to go again over many topics which have been already 
well treated. 
Thus, it is needless that I should occupy your time by alluding to the 
engrossing subject of the most recent natural operations with which the geolo- 
gist has to deal, and which connect his labours with those of the ethnologist. 
On this head I wdll oi^ly say, that having carefully examined the detrital accu- 
mulations forming the ancient banks of the river Somme in France, I am as 
complete a believer in the commixture in that ancient alluvium of the works of 
man with the reliquiae of extinct animals as their meritorious discoverer, M. 
Boucher de Perthes, or as their expounders, Prestwieh, Lyell, and others. I 
may, however, express my gratification in learning that our own country is now 
affording proofs of similar intermixture both in Bedfordshire, Lincolnshire, and 
