THE GEOLOGIST. 
and barytes, do uot sliow such an increase of the point of fusion in conse- 
quence of pressure, and he is led to the belief that it is only in the more com- 
pressible substances that the law holds true. Independently of this, hoAvever, 
Mr. Hopkins points out to me that in the above calculation it is assumed that 
the conductivity of the rocks is the same at great depths as at the surface. lu 
opposition to this he has shown experimentally that the conducting power for 
lieat is at least twice as great for the dense igneous rocks as for the more 
superficial sedimentary formations of clay, sand, chalk, &c. And these close- 
grained igneous rocks are those which we believe must most resemble the 
rocks at great depths below the surface. Now Mr. Hopkins shows that if the 
conductive power were doubled, the increase of depth, corresponding to a 
given increase of temperatui'e, would be doubled, and we should probably have 
to descend 80 or 100 miles to reach a temperature of 3,000 degrees, besides the 
further increase which investigation may show to be due to the influence of 
pressure on the temperature of fusion. Mr. Hopkins therefore concludes that 
the extreme thinness of the crust assumed by some geologists to account for 
volcanic phenomena is untenable. Calculations on entirely independent data 
led him to conclude that the thickness did not fall short of 800, instead of 30 
or 40 miles. If it be so much, he is further led to believe that the super- 
ficial temperature of the crust is due to some other cause than an internal 
fluid of nucleus. It remains a problem, therefore, which my friend, Mr. 
Hopkins, is endeavouring to solve, as to what is the actual condition of the 
earth at great depths, and the relation of terrestrial heat to volcanic pheno- 
mena. 
Mr. W. Hopkins considered the paper of Dr. Fairbairn merited more con- 
fidence than any which had before been given to the public ; for no previous 
communication had so largely taken into account the various circumstances in 
connection with deep mines, which bore on the temperature of the earth's 
crust. The condition of the rocks and walls, as well as the water in mines, 
must necessarily have a varj'ing effect upon the temperature ; and these facta 
had not previously received sufficient attention at the hands of those who had 
made experiments. One great advantage of the experiments recorded by tlie 
President of the Association was that they were made in a mine before it had 
been worked. The strata of Dnkinfield mine were very much inclined, and 
tliere was a good deal of water in it. For this reason great caution was needed 
in working it, because a wet mine gave a higher degree of temperatm-e than a 
dry mine. Hitherto there had been great difiiculty in making obseiwations and 
experiments in mines. Dr. Tairbairn supposed that 3,000 degrees miglit be the 
temperature of fusion. It might be greater for all they knew. He should 
be inclined to tliink it was greater. A thickness of 800 or 1,000 miles for the 
crust of the earth was more consistent with his own observations, and he 
positively insisted on a greater depth than 100 or 200 miles. 
VELOCITY OF EAETHQUAKE- WAVES. 
By R. Mallet, Esq., F.G.S. 
The experiments which were conducted at the large blasting-operations at 
Holyhead were imdertakeu at the joint request of the Royal Society and the 
British Association. Mr. Mallett confessed that a few years ago he- was 
under the impression that the velocity of the wave-particle of an earthquake, 
though not the same as the wave-transit, was nearly equal to it. It was only 
