March 22, 1888] 



NATURE 



489 



forward, the whole heap always showed a tendency to rise 

 and ride forward e7i masse over the less disturbed beds in 

 front. Fig. 2 shows a typical section produced at this 

 more advanced stage of the movement. This new plane 

 of shear may be called a "major thrust," as distin- 

 guished from the "minor thrusts" shown in Fig. ', 

 and in the upper part of this figure. The structure of 

 these artificial rock-masses bears a remarkable resem- 

 blance to that of the great thrust areas of Sutherland 



^ 



Fig. I. 



and Ross. Everywhere along that great region of earth 

 movement major thrust planes are found truncating 

 sets of minor thrusts, just as has taken place in this 

 experiment. The extraordinary heaping up and local 

 thickening of Silurian strata, and the superposition across 

 their upturned edges of 'iuge slices of Archaean gneiss 

 and Cambrian sandstone, are phenomena which, before 

 the thrust-plane theory had been originated, were quite 

 inexplicable.^ 



Fig. 2. 



The second series of experiments was intended to 

 ascertain how such great thrusts might have originated, 

 and to trace their connection with folds and great terres- 

 trial movements of upheaval and mountain building. 



Stratified beds, similar to those employed before, were 

 formed on a band of stout wax-cloth, about 2\ feet long, 

 and 7 inches broad, secured at the ends to vertical blocks 

 of wood. "When pressure was applied to the ends, the 

 wax-cloth was thrown into folds, but the folds did not 



■ Fig. 3. 



in all cases reach the surface, but found relief in thrusts, 

 as shown in Fig. 3. 



In this experiment an anticline was first formed at the 

 end of the wax-cloth nearest the pressure. A thrust 

 appeared at the surface, and, on examining the section, 

 this was found to bend down and bury itself in the left 

 monocHnal member of the fold. A second anticline was 



I The effect of major and minor thrusts is well seen in the section of the 

 Durness and Eriboll district above the map in the second edition of Dr. A. 

 Geikie's " Scenery of Scotland." 



ne.xt started in advance of the first, and, on continuing 

 the push, a second thrust, similarly situated with regard 

 to the underlying fold, was produced. By this means it 

 may be possible to explain how thrusts are connected 

 with movements of deep-seated parts of the earth's crust, 

 and also how, as in the Highlands, they occur over broad 

 areas all inclined in the same general direction. If this 

 section affords the true explanation of their origin, it is 

 clear that thrusting is only a surface phenomenon, and that 

 the complex structures of the North-West Highlands are 

 structures which can only originate at the outer edge of 

 a great mountain-system of elevation. 



Fig. 4 represents a section produced with the same 

 apparatus, but here the pressure was applied from both 

 sides. An anticline was started at the centre of the 



wax-cloth, and as the pressure was continued the strata 

 were squeezed into a form closely resembling that known 

 as " fan structure." Two small arches were next formed, 

 one on each side of the original fold, and the pressure 

 was continued. A second fan made its appearance out- 

 side the first, and at each side there was a tendency for 

 thrusts to be produced, as shown in Fig. 5. Throughout 

 the experiment the lowest stratum of damp sand next 

 the wa.x-cloth was compressed and distorted, till, at the 

 last stage of the movement, it became very much " staved 

 together " above the synchnal folds of the wax-cloth on 

 either side, and was completely "nipped out" at the 

 crown of the central fold. During the movement in the 

 mass it was, in fact, made to flow like a viscous body, 

 along a series of approximately vertical planes, which in 



Fig. 5. ; 



Nature would be described as planes of foliation. This 

 experiment, then, may help to explain not only the origin 

 of the fan structure of the Alps, &c., but also the com- 

 mon occurrence in the centre of the fan of a core of 

 crystalline rock with vertical foliation. 



The experiments of the third series were modifications 

 of those of Prof A. Favre,of Geneva (see NATURE,vol.xix. 

 p. 103), who covered a band of stretched caoutchouc with 

 beds of adhesive clay, and on allowing the elastic sole 

 to contract, observed the wrinkling up of the surface of 

 the clay into a series of miniature Alpine ridges. The 

 author modified Favre's experiments by separating the 

 upper and lower portions of the clay with sheets of 

 paper, so that the former could be stripped off at the end 

 of the experiment without disturbing the lower part of 



