346 MR CADELL ON EXPERIMENTAL RESEARCHES IN MOUNTAIN BUILDING. 



depends on the direction of pressure, and is practicably independent of the direction of 

 dip of the beds. 



A bed of soft tenacious cla}^ (figs. 9-11) was arranged below brittle strata of damp 

 sand niixed with a little stucco powder, and a bed of black foundry loam well packed 

 together. The object of the experiment was to test the behaviour of brittle beds on a 

 thoroughly plastic base when compressed longitudinally. The three figures show the 

 structure at as many different stages of compression. The result is certainly surprising, 

 as in each case the plastic clay, as well as the stiffer beds above, has undergone faulting. 

 It is to be observed, that as the compression proceeds the thrust-strata on the left of 

 the anticline bow forward, and tend to approach vertically, while the thrust-planes 

 originally inclined to the right are bent over to the left also. The experiment also 

 shows how folds are built up of interstitial displacements which may at places become 

 divided out along a few lines. At each stage, when the section was examined, about 

 an inch was pared off the edge, so that each figure shows the section a short distance 

 further along the strike of the beds than in the preceding figure. In fig. 10, the left 

 limb of the anticline is built up of many small distinct thrust-planes ; but in fig. 1 1 the 

 same member is shown as a single curve, proving that folding may pass into faulting 

 a short distance alons the strike of the distorted strata. 



i , 2b 



Fig. 12. 



In fig. 12 the tendency of the thrust-planes and beds to bow forward on advancing 

 is well exhibited. This is due to the resistance in front caused by friction below, 

 coupled with the staving together of the materials in more immediate proximity to the 

 pressure board. There is, in short, evidence of a continued attempt at the formation of 

 an anticline just in front of the region of maximum pressure. As in former experiments, 

 the beds on the right limb of the incipient fold are attenuated very considerably, and 

 show the wavy faulted structure already alluded to. The bending down of the thrust 

 beds on the left, it need scarcely be said, is thus a deceptive appearance, as the front 

 has remained quite stationary, while it is the bach portion which has been wedged up. 



In all the experiments of this series there was a tendency on the part of the 

 pressure board to rise and move obliquely forwards and upwards as the heaping up 

 progressed. In next experiment, instead of holding the pressure board in a constrained 

 vertical position, with its lower end against the fixed sole, a cushion of sand lecaa 

 substituted for it, and the push applied behind. The result, as was anticipated, showed 



