iv Sir James Hall 191 



in fusing limestone under pressure, he could adduce in that 

 single result "a strong presumption in favour of the solution 

 which Dr. Hutton has advanced of all the geological 

 phenomena ; for the truth of the most doubtful principle 

 which he has assumed has thus been established by 

 direct experiment." 1 



Hardly less striking were Hall's experiments in illustra- 

 tion of the processes whereby strata, originally horizontal, 

 have been thrown into plications. His machine for contort- 

 ing layers of clay is familiar to geological students from the 

 illustrations of it given in text-books. 2 He showed how 

 closely the convolutions of the Silurian strata of the Ber- 

 wickshire coast could be experimentally imitated by the 

 lateral compression of layers of clay under considerable 

 vertical pressure. In this, as in his other applications of 

 experiment, he led the way, and laid the foundation on 

 which later observers have built with such success. 



There was thus established at Edinburgh a group of 

 earnest and successful investigators of the history of the 

 earth, who promulgated a new philosophy of geology, based 



1 " Account of a series of experiments showing the effects of compression 

 in modifying the action of heat," read to the Royal Society of Edinburgh, 

 3rd June 1805. Trans. Roy. Soc. Edin. vi. p. 71. The same ingenious 

 observer subsequently instituted a series of experiments to imitate the 

 consolidation of strata. By filling an iron vessel with brine and having 

 layers of sand at the bottom he was able to keep the lower portions of the 

 sand at a red heat, while the brine at the top was not too hot to let the 

 hand be put into it. In the end the sand at the bottom was found com- 

 pacted into sandstone. Op. cit. x. (1825), p. 314. 



2 Trans. Roy. Soc. Edin. vol. vii. p. 79 and Plate iv. As already re- 

 marked, Hall differed from his master and from Play fair in regard to their 

 views on the efficacy of subaerial denudation. He preferred to invoke 

 gigantic debacles, and to these he attributed the transport of large boulders 

 and the smoothing and striation of rocks, now attributed to the action of 

 glaciers and ice-sheets. 



