Professor Geihie on the " Pitchstone " of Eskdale. 223 



Huttonian Theory, Hall is found reflecting on the objections 

 made to Hutton's views, that had granite, porphyry, and 

 whinstone ever been in a fused condition, they would have 

 resembled glass, instead of presenting the stony crystalline 

 aspect in which they are now found. It occurred to him 

 that their present structure might be due not merely to the 

 effect of consolidation under pressure as Hutton supposed, 

 but more especially to extremely slow cooling. The occur- 

 rence of an accident at the Leith Glass Works supplied him 

 with an actual corroborative example — a large pot of glass 

 having cooled down slowly into a mass which had completely 

 assumed a stony structure. Convinced that this was a true 

 cause of change in the internal constitution of fused masses 

 he communicated his views to the Eoyal Society of Edin- 

 burgh in 1790,* and determined to pursue the subject by 

 instituting a series of experiments. On making known his 

 intention to Hutton, however, he received but little encour- 

 agement from him. Hutton judged that the degree of heat, 

 and the scale on which it had been manifested in nature, were 

 such that no experiments could be expected to bring any real 

 elucidation of the subject. As he afterwards expressed the 

 idea in his reply to the strictures of Kirwan, there are some 

 " superficial reasoning men who judge of the great operations 

 of the mineral kingdom from having kindled a fire and looked 

 into the bottom of a little crucible."f 



But, though unconvinced, Hall, out of deference to his 

 master's feelings, forbore to put his design into execution. 

 The year after Hutton's death, however, he gave to the Eoyal 

 Society of Edinburgh his first great paper — an essay which 

 must be regarded as one of the landmarks of science, for it 

 really laid the foundations of experimental geology.J He 

 proved by a series of ingeniously contrived and carefully 

 executed experiments that the internal structure of igneous 

 rocks has been in great measure determined by their rate 

 of cooling ; that where they congealed rapidly from a state 

 of fusion they appear quite glassy, while, where they were 



* Op. cit., iii., part i., p. 10. f Theory of the Earth, i,, p. 251. 



X Experiments on Whinstone and Lava (read March 5 and June 18, 1798). 

 Trans Roy. Soc. Edin.^ v., p. 43. 



