178 F. D. ADAMS EXPERIMENT IN GEOLOGY 



types of structure is determined, in the first place, by the petrographical 

 character of tlie sediments within the complex — that is, their relative 

 hardness — and their position in relation to one another. In the second 

 place, by the character of the basement on which the sedimentary strata 

 lie, andjthirdly, by the amount of vertical loading to which they are sub- 

 jected during the action of the tangential thrust. 



Paulcke's work thus in certain directions develops the principles dis- 

 covered by Bailey Willis and represents the last of a series of experimental 

 studies which have throT\Ti much light on the mechanism of mountain- 

 making. 



But while much light was being thro^^Ti on the mechanics of mountain- 

 building by the experimental method, another and extremely obscure, 

 but highly interesting, group of phenomena was attracting attention, 

 namely, the movements and changes which take place within the sub- 

 stance of the rocks themselves when subjected to these enormous forces 

 by vv^hich they are folded into mountain chains. The results of these 

 forces are seen in their most striking forms in those rocks which have 

 been buried in the deeper parts of the earth's crust, where these forces act 

 most intensely. The phenomena are those of schistosity or foliation, 

 rock-flow, and the accompanying aspects of metamorphism. 



The schistose, foliated, or gneissic structure displayed by these rocks 

 was formerly regarded as an imperfect or partially obliterated bedding. 

 Fox,^* however, in 1837, claimed to have determined experimentally that 

 a current of electricity passed through damp clay would render the mass 

 schistose, and on the basis of this observation for a time many of the lead- 

 ing geologists looked to electric charges passing through the earth's crust 

 as a probable cause of the development of schistosity. Sorby,^^ however, 

 on the ground of a microscopic study of these schistose rocks, pointed out 

 that movements under pressure determined the development of this pecu- 

 liar structure and showed experimentally by submitting a stiff mass of 

 micaceous iron ore and clay to heavy pressure that in such a mixture 

 movements under pressure would produce a distinct foliation. 



A number of other investigators followed up this line of experimental 

 work, among whom may be mentioned Tyndall, Daubree, and Tresca, and 

 showed that under such movements in plastic masses any scaly mineral 

 present will become orientated in the direction of the movement, giving 

 rise to a schistose structure in the mass. T^TidalP^ even showed that in 



i*Mem. Royal Cornwall Polytechnic See., 1837. 



1° Edinburgh New Phil. Jour., vol. Iv, 1853, p. 437, and London. Edinburgh, and Dublin 

 Phil. Mag., vols, xi and xii, 1856. 



16 Comparative view of cleavage of crystals and slate rocks. Royal Institution of 

 Great Britain, .Tune 5, 1856. 



