BRITISH ASSOCIATION MEETING. 
435 
and with L. Yon Buch, Elie de Beaumont, and a host of geologists abroad, I 
had long ago arrived in the field. I, therefore, re-echo their voices in repeating 
the words of Mr. W. Harcourt, " that we are not entitled to presume that the 
forces which have operated on the earth's crust have always been the same." 
Looking to the only rational theory which has ever been propounded to account 
for the great changes in the crust which liave taken place in former periods — 
the existence of an intense central heat which has been secularly more and 
more repressed by the accumulation of sediment until the surface of the planet 
was brought into its present comparatively quiescent condition — our first General 
Secretary has indicated the train of causes, chemical and physical, which resolve 
some of the difficulties of the problem. He has brougiit before us, in a compen- 
dious digest, the history of the progress which has been made in this branch of 
our science, by the writings of La Place, Fourier, Von Buch, Fournet, and 
others; as well as by the experimental researches of Mitscherlich, Berthier, 
Seuarmont, Daubree, Deville, Delesse, and Durocher. Illustrating his views 
by reference to chemical changes in the rocKS and minerals of our own country, 
and fortifying his induction by an appeal to his experiments, he arrives at the 
conclusion, that there existed in former periods a much greater intensity of 
causation than that which now prevails. His theory is, that whereas now, in 
the formation of beds, the aqueous action predominates, and the igneous is only 
represented by a few solfataras, in the most ancient times the action was much 
more igneous, and that in the intermediate times fire and water divided the 
empire between them. In a word, he concludes with the expression of tlie 
opinion, which my long-continued observatiou of facts had led me to adopt, 
" that the nature, forces, and progress of the past condition of the earth cannot 
be measured by its existing condition." 
In addition to these observations on metamorphism, let me remind you that, 
on the recommendation of the British Association, other important researches 
have been carried on by Mr. William Hopkins, our new General Secretary, and 
in the furnaces of our President, Mr. Fairbairn, on the conductive powers for 
heat in various mineral substances. Although these experiments have been re- 
tarded by a serious accident which befel Mr. Hopkins, they are still in progress, 
and I learn from him that, without entering into any general discussion as to 
the probable thickness of the crust of our planet, we may even now affirm, on 
experimental evidence, that, assuming the observed terrestrial temperature to 
be due to central heat, the thickness of this crust must be two or three times 
as great as that which has been usually considered to be indicated by the 
observed increase of temperature at accessible depths beneath the earth's 
surface. 
Of the Devonian rocks or Old Red Sandstone, much might be said if I were 
to advert to the details which have been recently worked out in Scotland, by 
Page, Anderson, Mitchell, Powrie, and others ; and in England, by the re- 
searches of the Rev. W. Symonds, and other n^embers of the Woolhope and 
Malvern Clubs. But confining myself to general observations, it may be stated, 
that a triple sub-division of that group, which I have shown to hold good over 
the Continent of Europe as in our own country, seems now to be generally 
admitted, whilst the history of its southern fauna in Devonshii-e has recently 
been graphically and ably elaborated by Mr. Pengelly, in. a paper printed in 
our last volume. 
In Herefordshire and Shropshire the passage of the upper members of the 
Silurian rocks into the inferior strata of the Old Red group, has been well 
shown by Mr. Lightbody, and the fossils of its lower member have been vigo- 
rously collected. Whilst in Scotland Mr. Geikie and others have shown the 
upward passage of its superior strata into the base of the Carboniferous rocks ; 
and Dr. Anderson announces the finding of shells with Crustacea in the lower 
or grey beds, south of the Tay. I may' here note, that the point which I have 
