350 MR CADEIX ON EXPERIMENTAL RESEARCHES IN MOUNTAIN BUILDING. 



side of the section. As in the former case, the brittle beds at the surface showed signs 

 of gifang way without folding. 



All the strata to the rig-lit of the new fold were now in motion, and movement 

 along the first thrust-plane ceased. The pressure found relief along the second thrust 

 only, and the anticline beneath was squeezed together till the sides nearly met (fig. 17). 

 Had the waxcloth been longer, a third anticline would have arisen in advance of 

 No. 2, and so on for others, each giving rise to a thrust at the surface of the section. 

 This 1 conceive may explain how thrust-planes are found across great stretches of 

 country, all lying at the same angle and dipping in the same general direction. 



Fie. 18. 



In this experiment (fig. 18) the beds were of sand, with brittle stucco lamina? between. 

 The waxcloth was compressed from both sides equally, but to prevent complications, 

 three small anticlines were started by wedging it up at the centre and near the sides. 

 The section is interesting, as showing clearly the anatomy, so' to speak, of the structure. 

 The fold at the right shows the beds becoming increasingly curved from below upwards, 

 till they have no longer been able to bear the strain, but have snapped, and as before 

 found relief along a thrust-plane. It is to be noted that the upper beds underwent thrust- 

 ing simultaneously with the folding below. According to Prof. A. Heim's well-known 

 theory, that reversed faults are produced by monoclinal folds giving way along the 

 middle limb, we should have expected to find traces of a fold having first been formed 

 not only below, but all along the line of shear. These and former experiments show that 

 Heim's theory, which is no doubt true in many cases, is by no means of universal appli- 

 cation. Thrust faults may originate at once, without passing through an initial stage of 

 overfojding. 



The section is also interesting as showing how one thrust-plane may overlap 

 another running in the opposite direction. In the right half of the section, two thrusts 

 are seen overlapping in this way, so as to repeat the upper dark layer three times in the 

 same vertical line. 



Figs. 19-22« show the results of experiments designed to imitate the type of 

 mountain building found at many parts of the Alps, and known as " fan structure." 



As a symmetrical section was desired, the strata on the waxcloth, which consisted of 

 sand and stucco laminae, were slightly raised in the centre. When pressure was applied, 

 this arch rose up into a single anticlinal fold. As the side pressure was continued, the 

 limbs of the anticline were squeezed together till the beds slightly passed the vertical, 

 and began to dip inwards at high angles, exhibiting; the desired fan-like arrangement. 



