MR CADELL ON EXPERIMENTAL RESEARCHES IN MOUNTAIN BUILDING. 343 



of the great thrust-planes in Sutherland, where a similar process of heaping up of 

 Silurian quartzites, shales, and limestones has at places given these strata an abnormally 

 thick appearance, which, before the discovery of the thrust-planes, was quite inexplicable. 

 For this arrangement of strata I would propose the name of" wedge structure." 

















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Fig. 4. 



Fig. 4 re. 



These strata (fig. 4) had originally a length of 4^ feet and a depth of 4^ inches. The 

 brittle beds are near the bottom of the section, while the upper layers consist of damp 

 sand, whose particles could undergo some interstitial movement, and thus allow it to 

 behave like a partially plastic body. On compressing the end 8 inches, the surface was 

 observed to swell up in front of the pressure board. To all outward appearance, the 

 result did not differ from that obtained in the experiments of Pfaff already alluded to, 

 and figured in his Mechanismus der Gebirgsbildung , p. 23. In examining the section, 

 however, the brittle beds below were seen to have snapped as in former experiments, and 

 given way along a single shear plane, without any folding. Towards the surface this line 

 of shear is seen to split up, till the movement, which was confined to one plane below, 

 has become so distributed throughout the mass that the underlying thrust-plane is lost 

 in a great fold above, and never appears at the surface. The main point of interest here 

 is the passage of a fault below into an anticlinal fold at the surface of the ground. 



Fig. 5. 



Fig. 5«. 



On continuing the push 4 inches farther, the anticline was slightly compressed, and 

 a second reversed fault was started in advance of the first (fig. 5), just as was observed to 

 take place in figs. 1 and 2. 



