THE MECHANICS OF GEOLOGIC STRUCTURES 507 



enables one to judge of the manipulation necessary to secure the 

 thickness desired. The chilling can be accomplished by allowing 

 cold tap water to flow over the uncoated side of the rubber. If 

 chilled too rapidly or too much, cooling cracks will develop in the 

 parafhn. Deformation of the rubber sheet produces systematic 

 fractures in the paraffin bearing definite relations to the manner 

 of deformation. 



Folds are produced by coating the paraffin with a thin layer of 

 plastic wax and laying smoothly over this a very thin sheet of 

 rubber, such as is used by dentists, or a sheet of tinfoil. When 

 the rubber thus coated is deformed, folds are developed in the 

 plastic wax and its coating, as described later. 



This type of apparatus has an advantage over the deformation 

 of large masses of material in a compression machine of the piston 

 type, as the forces are transmitted through the rubber and there- 

 fore applied at every point in the coating. The thin coat of 

 paraffin may be considered as representing the flat-lying* rocks in 

 the zone of fracture, having wide lateral extent as compared with 

 thickness. The sheet of rubber might correspond to the deeper 

 zone of rock flowage. Deformation of the paraffin-coated rubber 

 is comparable to deformation affecting the zone of flow and the 

 overlying zone of fracture simultaneously. A distribution of 

 stresses throughout the deformed mass is obtained. The phe- 

 nomena of repeated faults or extensive systematic joining and 

 of repeated folds much more closely simulate nature than do the 

 single fractures obtained in a testing machine or the single folds 

 which develop ahead of the piston in the type of apparatus employed 

 by Willis and others. 



An apparatus employing a stretched rubber sheet on which 

 plastic layers were built up and deformed by the contraction of 

 the rubber was employed by Alphonse Favre^ in connection with a 

 study of rock deformation. In order to apply the compressive 

 force he attached wooden blocks to the rubber to serve as buttresses 

 or thrust blocks. The net result was not essentially different from 

 the results obtained by later investigaters employing apparatus of 



^ Alphonse Favre, Archives des Sciences Physiques el Naturelles, Nouv. Per., 

 Tome 62 (Geneve, 1878), pp. 193-211. 



