522 WARREN J. MEAD 



produced by allowing the thick rubber sheet to contract. This 

 resulted in the development of the system of folds illustrated in 

 Figure ii. This figure represents a positive reproduction of the 

 surface of the specimen in plaster of Paris. The original specimen 

 was not of a nature to be easily photographed. This experiment 

 and others of the same type show a set of folds striking in a direc- 

 tion at right angles to the direction of shortening. All of these 

 folds pitch at the ends and disappear. Depending on the thickness 

 of the wax and the behavior of the rubber sheet, shortening is 

 accomplished by a few large folds or by a larger number of smaller 

 folds. An interesting overlapping of the folds is noted. When- 

 ever a fold terminates by pitching, another fold appears over- 

 lapping it and continuing the necessary amount of shortening. 

 The experiments demonstrate very well that pitching folds do 

 not necessarily mean cross-folding but that they are developed 

 in flat-lying beds with perfectly even application of shortening 

 stresses. 



Development of folds under conditions of shear or rotational 

 stress. — -The apparatus used is illustrated in Figure 4, except that 

 in place of a thin layer of paraffin, a very thin layer of plastic 

 wax was applied to the very tightly stretched rubber sheet and 

 over this a sheet of tinfoil was carefully spread, care being taken 

 to secure perfect adhesion of the tinfoil to the underlying wax. 

 Deformation was accomphshed by causing the slidable clamp to 

 move parallel to the other clamp by means of the screw. This 

 resulted in a set of folds in the tinfoil- covered wax layer, shown 

 in Figure 12. The illustration is from a photograph of a posi- 

 tive reproduction of the specimen in plaster of Paris. The direc- 

 tion of shearing forces is shown by the arrows and the amount of 

 deformation by the shape of the parallelogram which was originally 

 rectangular. The folds, it will be noted, have their axes parallel 

 to the direction of elongation of the mass. All of them are pitch- 

 ing folds. They illustrate the phenomenon of repeated folds. 

 Like the previously described experiment they demonstrate that 

 pitching folds do not necessarily mean cross-folding or shortening 

 in the direction of the axes. As a matter of fact, in this experi- 

 ment tensional stresses existed in the direction of the axes of the 



