APPENDIX— HALL'S EXPERIMENTS. 137 



If Hutton, however, proposed the theory of the unstra- 

 tified rocks, it was left for Sir James Hall to establish its 

 truth, by a series of original experiments which will make 

 his name ever be respected by the scientific world, and ever 

 have a prominent place in the page of geological history. To 

 this distinguished examiner, the crystalline structure of 

 granite, porphyry, and trap, appeared at first to contradict 

 the hypothesis of Dr Hutton ; but the experiments which he 

 made with such success, convinced him that the effects of 

 compression were of a gigantic character, and that dif- 

 ferences in it might account for the dissimilarity of aspect 

 which exists between a trap or a Plutonic rock, and a lava 

 or a Volcanic rock As the experiments of Sir James Hall 

 on " Whinstone" (Trans, of Royal Society of Edin. v. 5.) 

 were made on several well known rocks in the neighbour- 

 hood of Edinburgh, which we have already described, we 

 shall briefly extract, from his paper in the Transactions of 

 the Royal Society, a few particulars in regard to this inte- 

 resting subject. 



The first trap-rock which was examined was the green- 

 stone of Bell's Mills, a point which, in its proper place, we 

 have already noticed. A black-lead crucible filled with 

 fragments of this stone was introduced into the reverberat- 

 ing furnace of an iron foundry ; having remained there a 

 quarter of an hour, the greenstone became completely fused 

 and was agitated with a violent ebullition, and on being 

 removed from the furnace and allowed to cool rapidly, was 

 found to have lost its crystalline arrangement, and to have 

 assumed a vitreous aspect. It was thus found that fused 

 greenstone when quickly cooled has not a granular structure. 



In subsequent experiments, Sir James Hall endeavoured, 

 by slow and gradual cooling, to cause the trap to assume its 

 original characters. The first attempts which were made 



