MR CADELL ON EXPERIMENTAL RESEARCHES IN MOUNTAIN BUILDING. 341 



are the edges of the plaster of Paris laminae separating thicker layers of damp sand. 

 These white beds were often so hard as to offer much resistance to the paring knife, and 

 great care was required to avoid tearing the material out in cakes, and leaving cavities 

 between the softer sand beds. 



Fig. 1. 



In this experiment (fig. 1) the strata had a thickness of about 4 inches, and consisted 

 of damp sand with three thin bands of stucco. As soon as pressure was applied, the 

 material immediately in front of the pressure board began to swell up into an anticline in 

 exactly the same way as Pfaff's strata of loam and papier-mach^ pulp were observed 

 to do. The right limb of the anticline being pressed in, gradually assumed a vertical 

 position parallel to the face of the pressure board. At this stage the pressure from the 

 right was uniformly distributed from the crown of the arch downwards. But the 

 resistance on the left was only exerted as far up as the level of the surface of the 

 undisturbed strata, so that the part of the arch above this level was free to travel forwards. 

 It did so for a short space, and produced the monocline at the top of the section. Had 

 the upper strata been more rigid, the} r would not have become bent into such a form, but 

 would have snapped at once, and formed a reversed fault, as has indeed been done in the 

 thin bed immediately at the surface. It was, however, impossible for the upper part of 

 the anticline to move far forward with an increase of pressure from the right, since all 

 particles in the same vertical plane were subjected to equal pressure from that quarter. 

 In the low r er part of the section, the horizontal pressure from the right was met by the 

 horizontal longitudinal resistance of the strata combined with the vertical statical 

 resistance of the sole, so that the resultant force tended to shear the strata obliquely 

 along a series of planes inclined towards the right. The stiffness of the beds now came 

 into play, and prevented this shearing strain from being distributed throughout the 

 mass. Instead of this, the brittle strata snapped at one point, and all the movement was 

 concentrated along the line of weakness thus produced. The whole mass above this thrust- 

 plane moved obliquety upwards and forwards, and all interstitial movement ceased. 

 The thrust-plane or reversed fault did not start directly from the bottom of the 

 pressure board, but met the fixed sole a short distance in advance. As soon, how- 

 ever, as the front of the pressure board reached the point of the wedge of undisturbed 



