500 R. T. CHAM BERLIN AND F. P. SHEPARD 



The first model was compressed without, covering. Since the 

 material was quite plastic, a high overfold developed. This suggests 

 that horizontal compression near the surface of the earth, where the 

 overburden is not great, would be likely to produce large simple 

 folds, rather than smaller complex folding. 



In the second experiment the layer was covered with sand only, 

 so that there were not more than lo or 15 pounds of total overbur- 

 den, uniformly distributed over the model whose dimensions were 

 2o"x5f x^". In this case the folding was more complex. The 

 principal anticline developed nearer the pressure block; did not go 

 so high as in the previous experiment; was subjected to a series of 

 minor undulations ; and in addition, a small thrust fault developed. 



In the third experiment the overburden totaled 50 pounds, 

 uniformly distributed. In this case the folding did not rise quite so 

 high as in the second, but was more distributed along the length of 

 the model A thrust fault which developed on one side of the model 

 had a greater displacement than that in the second experiment. 



In the fourth experiment a weight of 190 pounds was placed over 

 the model. The results of compressing this model were quite differ- 

 ent from the others. There was only a very minor amount of fold- 

 ing, but on the other hand, faulting was important, and there was 

 some thickening of the layer. The deformation was not confined 

 to any particular zone. 



Thus the general result of increasing overburden was a tendency 

 to produce more complex and more distributed folding, more fault- 

 ing, and more thickening of the material. Since it is easier for a 

 series of small folds to hold up a heavy covering than it is for one 

 large fold, it is not difficult to see why the folds should not go so high 

 when the overburden was greater. Similarly, in nature, folds should 

 probably be smaller and more widely distributed in depth than at 

 the surface. The occurrence of faults in the specimens where the 

 overburden was greatest is startling, as it is generally supposed that, 

 with increasing depth, folding gradually takes the place of faulting. 

 Faulting, however, is a much easier type of deformation under quick 

 acting stresses than folding, and may therefore be a more natural 

 way of taking up the shortening in cases where the resisting pressures 

 of overburden tend to prevent the upbowing necessary in folding. 



