.1,1 



J38 MB ( 'A DELL ON EXPERIMENTAL RESEARCHES IN MOUNTAIN BUILDING. 



far-reaching in their application, not only to the mountains of Scotland, hut also to the 

 Alps and other systems of mountains of elevation, I shall postpone the second or theor- 

 etical part of the paper till I have better studied the problems in mountain building to 

 which the experiments seem to afford in some measure a solution, and confine any 

 ivmarkson the experiments to a bare enumeration of some of the more obvious inferences 

 from these results. 



Description of Experiments. 



In experimenting on the behaviour of stratified rocks when subjected to horizontal 

 pressure, it has been usual to regard great rock masses as practically plastic, and to 

 imitate their plications with correspondingly plastic substances. Sir James Hall, the 

 father of experimental geology, describes, in an early volume of this Society's Transac- 

 tions* how he imitated the foldings of the Silurian strata on the Berwickshire coast by 

 compressing pieces of cloth or layers of clay. Prof. A. FAVRE,t to whose interesting 

 experiments allusion will afterwards be made, produced miniature alpine ridges with 

 laminae of fine clay, and Prof. F. PfaffJ tried the effect of horizontal pressure on layers 

 of loam and papier-mache pulp. Daup>ree,§ the greatest living exponent of experimental 

 geology, departed, however, from the old paths by using a prism of wax which was 

 flexible within certain limits only, and snapped on the application of greater pressure, 

 producing a series of little reversed faults. Mellard Reade, in his recent valuable work 

 on the Origin of Mountain Ranges, describes a variety of experiments, some of which 

 resembled those of Pfaff's, and yielded somewhat similar results. He used strata of clay 

 with lubricated surfaces. 



When the correct interpretation of the structure of the North- West Highlands was 

 arrived at, however, it soon became evident that the rocks in that area had in many cases 

 behaved like brittle rigid bodies, which, instead of undergoing plication when subjected 

 to horizontal compression, had snapped across and been piled together in great flat 

 slices like so many cards swept into a heap on a table. 



To imitate such phenomena, it was therefore necessary to employ materials of such a 

 kind that they would, when compressed horizontally, snap and give way in definite 

 directions rather than bend into folds like plastic bodies. 



The idea occurred to me that plaster of Paris, inters tratified or mixed with layers of 

 sand, might satisfy the requirements of the case. After several failures, this plan was 

 .successful. The dry stucco powder was spread in thin layers between thicker beds of 

 damp sand of different colours, and in a few minutes it had absorbed enough moisture from 

 the porous strata to permit of partial hydration. It "set" into hard brittle lamina'. 

 which usually snapped under strain, but in some instances permitted folding to take 

 place. In some of the experiments black foundry loam was used, and when well damped 

 and packed together, proved an excellent material with which to imitate rock-strata, as 



* Trans. Roy. Sue. E'Hii., vol. vii. p. 85. t Nature, xix. p. 103. 



J Miihitiii.<ini::< il: i- (,'<'l>i njshilihi iiij, p. 23. § G&loyie experimentalc, p. 321. 



