174 P. D. ADAMS EXPERIMENT IN GEOLOGY 



have been reduced by heat into a state of fluidity ; thus the saline or finer 

 kinds of marble that have a structure highly crystallized must have been 

 softened to a degree little short of fusion before this crystallization could 

 take place," and that he believed it to be possible that "calcareous earth 

 under great compression may have its fixed air retained in it, notwith- 

 standing the action of intense heat, and may by that means be reduced 

 into fusion or into a state approaching it.'* 



Here was a well defined issue between the two great camps into which 

 the geological world was then divided. On its outcome depended the 

 explanation of the nature and origin of the rocks constituting a large 

 portion of the earth's crust. They could not settle the dispute by obser- 

 vation in the field, but it was triumphantly decided by recourse to experi- 

 ment. To Sir James Hall,^ of Edinburgh [1761-1832], who was an 

 intimate friend of Hutton, is due the credit of having carried out this 

 epoch-making experimental demonstration. 



Taking first the "whinstone," or intrusive olivine basalt of the district 

 about Edinburgh, and later the lavas from Vesuvius and Etna, he fused 

 them in a reverberating furnace and obtained from the fused material 

 by rapid cooling a perfect glass. When, however, the rock was fused and 

 cooled more slowly, he obtained a product which, while not like whinstone, 

 was of an intermediate character, like the 'T.iver of an animal," to use 

 his own quaint expression, and often containing "a multitude of little 

 spheres having a dull -or earthy fracture." This we now know to be a 

 glass filled with devitrification products, and he makes some interesting 

 observations with reference to the sudden hardening of the glass, even if 

 the temperature remains constant, as it passes into this devitrified con- 

 dition. 



Finally he found that if the fused mass was cooled very slowly, during 

 a period of several hours, he obtained "a substance differing in all respects 

 from glass and in texture completely resembling whinstone." 



He thus demonstrated experimentally that, contrary to the opinion of 

 the i^eptunists, a crystalline rock might be produced by the cooling of a 

 fused magma and thus be of igneous origin. 



Hall* then carried on a long series of most brilliant, interesting, and 

 ingenious experiments, in which he submitted powdered chalk to a red 

 or white heat in closed gun-barrels or porcelain tubes. A portion of the 

 chalk was thus disassociated, while the remainder was submitted to a 

 high pressure by the carbonic acid gas thus produced. The manner in 



3 Experiments on whinstone and lava. Trans. Roy. Soc. of Edinburgh, vol. v. ISO"). 

 * Sir .Tames Hall : Account of a series of experiments showing the effect of compres- 

 sion in modifying the action of heat. Trans. Roy. Soc. of Edinburgh, vol. vi. 1812. 



