TRANSACTIONS OF THE SECTIONS. 101 



gists abroad, I had long ago aiTivcd 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 cnist have always been the same." 

 Looking to the only rational theory which has ever been propounded to accoimt for 

 the great changes in the crust which have taken pLace in former periods — the exist- 

 ence of an intense central heat which has been secularly more and more repressed 

 by the accumulation of sediment imtil the sm-face of the planet was brought^ into 

 its present comparatively quiescent condition — our first General Secretary has indi- 

 cated the train of causes, chemical and physical, which resolve some of the difii- 

 culties of the problem. He has brought before us, ia a compendious digest, the 

 history of the progress which has been made in this branch of our science, by the 

 vnitings of La Place, Fourier, Von Buch, Foumet, and others, as well as by the 

 experimental researches of Mitscherlich, Berthier, Senarmont, Daubree, DeviUe, 

 Delesse, and Durocher. Illustratiug his views by reference to chemical changes in 

 the rocks and minerals of our own covmtry, and foi-tifying his induction by an appeal 

 to his experiments, he arrives at the conclusion, that there existed in foiiner 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 ex- 

 pression of the opinion, which my long-continued observation of facts had led me 

 to adopt, " that the nature, force, and progi-ess of tjje past condition of the earth 

 cannot be meastired by its existing condition." 



Li addition to these observations on metamorphism, let me remind you that, on 

 the recommendation of the British Association, other important researches have 

 been earned on by Mr. William Hopldns) our new General Secretary, and in the 

 furnaces of our President, Mr. Fairbaim, on the conductive powers for heat in 

 various mineral substances. Although these experiments have been retarded by a 

 serious accident which befell 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 oiu* planet, we may even now afiinn, 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 temperatm-e 

 at accessible depths beneath the earth's surface. 



Of the Devonian rocks or Old Bed 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 researches of the 

 Eev. W. Symonds, and other members of the Woolhope and Malvern Clubs. But 

 confining myself to general observations, it may be stated that a triple subdivision 

 of that group, which I have shown to hold good over the Continent of Europe as 

 in our own comitry, seems now to be generally admitted, whilst the history of its 

 southern fauna in Devonshire has recently been graphically and ably elaborated by 

 Mr. Pengelly, in a paper printed in om- last volume. 



Li Herefordshire and Shropshire the passage of the upper members of the Silu- 

 rian rocks into the inferior strata of the Old Bed group has been well shown by 

 Mr. Lightbody, and the fossils of its lower members have been vigorously collected ; 

 whUst in Scotland Mr. Geikie and others have shown the upward passage of its 

 superiof strata into the base of the Carboniferous rocks; and Dr. Anderson an- 

 nounces 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 been for some j'ears endea- 

 vouring to establish as to the true position of the Caithness flags with their nume- 

 rous ichthyolites seems to be admitted by my contemporaries. The lamented Hugh 

 Miller considered these ichthyolites as belonging to the lower member of the group, 

 and had good gTounds for his views, since at his native place, Cromarty, these fish- 

 beds appear very near the base. But, by following them into Caithness and the 

 Orkneys, I have shown that they occupy a middle position, whilst the true base of 

 the group is the equivalent of the zone with Ceplialaspis, Pteraspis, and Ptet-y- 

 gottcs. 



And here it is right to state that the Upper Silurian rocks, which are clearly re- 



