252 EXPERIMENTS ON METAMORPHISM AND 



the Alps; by Sir R. Murchison and Darwin in the Andes; by Rogers* 

 in the Appalachian mountains, and by other geologists. 



There exists, however, between the position of the lamellce and 

 that of the beds, relations which Banr and Sharpe have with great sa- 

 gacity brought to light. 



The cause of the development of the foliated structure of the 

 phyllades has been attributed to crystalline, polar, or electric agen- 

 cies. These vague hypotheses could hardly find foundation but in 

 the fact announced by R. Fox, that wet clay in presence of electri- 

 cal currents will become perceptibly foliated, t These causes, which 

 may be qualified as occult, have, however, been adopted by eminent 

 savants, such as Sedgwick, Sir H. de la Beche,:): Sir J. Herschel,§ 

 Hopkins, II and Scheerer.^ This is easily explained, for the re- 

 markable fact which was more particularly calculated to lead to a 

 right explanation was only recently discovered by Baur.** This 

 geologist was the first to show, in his remarkable work, (1846,) that 

 the cleavage took place at the time of the convolvement of the beds, 

 and that it seems the result of a pressure normal to that which de- 

 veloped the convolutions. It was not until a year later that Sharpe, 

 to whom the priority has often been given, came to the same con- 

 clusion, founded on other very precise observations on the distortions 

 produced upon fossils. He afterwards established the same fact for 

 rocks in which no organic remains are found, ft 



The first attempt to imitate this phenomenon mechanically was 

 made by Professor Sorby, to whom we owe other ingenious researches. |J 

 He discovered by a microscopical examination of the disposition of 

 their elements, as well as by the compression of certain thin layers, 

 that the schistose rocks had undergone compression. Professor Tyn- 

 dall went still further ; he produced a foliated structure exactly like 

 slate in different plastic substances, as pipe-clay and wax, by com- 

 pressing and submitting them to a process of rolling. §§ It was thus, 

 without doubt, that this expert physicist was afterwards induced to 

 pay attention to the structure and movement of glaciers. 



I conclude by calling to mind that Laugel, engineer of mines, and 

 Professor Haughton have sought to subject to Calculation the effects 

 of pressure which have produced a schistose structure. 



■■■'" Proceedings of American naturalists and geohgisis ; 1845. 



f Report of the Cornwall Polytechnic Society ; 1837. Memoirs of the Geological Survey of Oreat 

 Britain, vol. i, p. 433. 



X Geological Report on Cornwall and Devon, p. 181 ; 1839. The polar forces, the author sup- 

 poses, are probably in connexion with terrestrial magnetism. 



§ Lyell. Manual of Geology, 5th ed., vol. ii, p. 448. 



[I On the connexion of geology with terrestrial magnetism. 



'^ Kardens Archiv.fur Mineralogie, vol. xvi, p. 109; 1842. Geological Observations on South 

 America, p. 168. 



^^ Karstens Archiv., vol. xx, p. 398; 1846. ■ ♦ 



ft Quarterly Review of the Geological Society of London, vol. iii, p. 74 ; 1847. Geological Pro- 

 ceedings, November, 1854. 



IJ In laminating clay, in which he had scattered pellicles of oxide of iron, Sorby noticed 

 that they placed themselves perpendicularly to the pressure. — {Edinb. Phil. Journal, 1853. 

 Quarterly Revieio, vol x, p. 73 ; 1854.) 



§§ Comparative view of the cleavage of crystals and slate rocks. — (Philosophical Magazine; 

 1856.) 



