THE GEOLOGIST. 



and barytes, do not 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, however, 

 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. In 

 opposition to this he has shown experimentally that the conducting power for 

 heat 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 temperature, 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 clue 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 varying effect upon the temperature ; and these facts 

 had not previously received sufficient attention at the hands of those who had 

 made experiments. One great advantage of the experiments recorded by the 

 President of the Association was that they were made in a mine before it had 

 been worked. The strata of Dukinfield mine were very much inclined, and 

 there 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 temperature than a 

 dry mine. Hitherto there had been great difficulty in making observations and 

 experiments in mines. Dr. Fairbairn supposed that 3,000 degrees might be the 

 temperature of fusion. It might be greater for all they knew. He should 

 be inclined to think 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 EARTHQUAKE-WAVES. 



By B. Mallet, Esq., F.G.S. 



The experiments which were conducted at the large blasting-operations at 

 Holyhead were undertaken 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 



