Geohsrical Address at Manchester. 409 



& 



lesse, and Duroclier. Illustrating his views by reference to chy- 

 mical 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 

 io-neous, 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 progress 

 of the past condition of the earth cannot be measured by its exist- 

 ing^ condition.' 



" In addition to these observations on metamorphism, let me 

 remind vou that, on the recommendation of the British Associa- 

 tion, 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 retarded by a serious accident which befel Mr. Hopkins, 

 they are still in progress, and I learn from him that, without en- 

 tering into any general discussion as to the probable thickness of 

 the crust of our planet, we may even now afiirm, 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 researches of the Rev. W. Sy- 

 monds 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 country, 

 seems now to be generally admitted, while the history of its 

 southern fauna in Devonshire has recently been graphically and 

 ably elaborated by Mr. Pengelley in a paper printed in our last 

 volume. 



