TEE MECHANICS OF GEOLOGIC STRUCTURES 521 



The apparatus of Willis permits deformation only by straight 

 shortening or compression and does not afford means of studying 

 the nature of folds developed by lateral shearing movements. It 

 seems probable that the movements between great earth masses 

 are in the nature of shears rather than simple straight-Hne com- 

 pression. In other words, the application of a compressive force 

 directly toward the point of maximum resistance would he less probable 

 than the development of a couple which would cause what has been 

 called a rotational stress. 



It does not appear to the writer that rocks have been folded 

 by forces transmitted to them in a manner at all similar to the 

 action of a piston against a more or less confined mass but that 

 shortening of the earth's crust has resulted from great compressive 

 forces extending to some depth, and that the fracture and folded 

 rocks within the zone of our observation have received from the 

 rocks beneath, in a large measure, the force which deformed them. 

 In other words, most of the faults or folds are the result of the 

 riding or dragging of the upper layers by the underlying materials. 



The writer has attempted to apply to the study of folds the 

 methods used in the study of fractures already described. The 

 two pieces of apparatus shown in Figures i and 4 were employed 

 but instead of the thin brittle coat of paraffin a very thin layer of 

 plastic wax (made by mixing beeswax and Venice turpentine) was 

 applied and over this, while the wax was still sticky, a sheet of 

 tinfoil was carefully spread. When the rubber sheet was allowed 

 to shorten, or was deformed by shear, the layer of wax and tinfoil 

 developed a series of folds. It was found that a very thin sheet of 

 rubber served the same purpose as the tinfoil. The purpose of 

 the thin sheet of rubber or tinfoil is to supply a layer with a cer- 

 tain small amount of competency. A layer of wax alone is entirely 

 incompetent and follows the deformation of the rubber sheet 

 without development of folds. A thin layer of tinfoil, sheet 

 rubber, or waxed paper supplies the element of competency which 

 results in the development of folds. 



Folds developed by pure shortening or compression. — The appa- 

 ratus shown in Figure 2 was employed, with a layer of plastic wax 

 covered by a sheet of thin dental rubber. The shortening was 



