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 have 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 brought 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, Eournet, and 

 others ; as well as by the experimental researches of Mitscherlich, Berthier, 

 Senarmont, 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 theoiw 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 the 

 opinion, which my long-continued observation 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. Eairbairn, 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 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 re- 

 searches of the Rev. W. Symonds, and other members 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 Devonshire 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 



