5o6 WARREN J. MEAD 



warping. A reverse fault implies conditions of shortening or 

 compression but may in addition to this possibly be an incident 

 in a general shearing movement, or_ a phenomenon of simple cross- 

 bending, or may be due to torsional warping. A series of folds 

 may be due to shortening or compression in a direction normal to 

 the trend of the folds or to a general shearing movement in a 

 direction at a considerable angle to the trend. In general the 

 shearing type of deformation has been largely neglected in analyses 

 of the mechanics of geologic structures, both fractures and folds. 

 It is in part the purpose of this paper to present the result of 

 experimental work which illustrates the variety of mechanical 

 explanations possible for a given structure, and ' incidentally to 

 emphasize the extent to which many of these may be related to 

 shearing, which the writer regards as an exceedingly common 

 type of deformation in rock movements. It is further purposed to 

 present and to illustrate experimentally analyses of the stresses 

 involved in the various types of deformation. 



DESCRIPTION OF APPARATUS 



Several years ago the writer devised for use in the structural 

 geology laboratory of the University of Wisconsin a simple type 

 of apparatus for studying and demonstrating relations of fractures 

 and of folds to the forces producing them. The apparatus is used 

 in three forms as illustrated in Figures i, 2, and 4. These are 

 similar in construction, consisting of a rigid rectangular frame of 

 gas pipe supporting two clamps between which a heavy sheet of 

 rubber is stretched. One or both clamps may be moved in various 

 ways by means of screws so that tension, compression, torsion, 

 and shear, or combinations of these may be produced in the rubber 

 sheet. 



The medium in which fractures are produced is a thin coating 

 of paraffin applied to the upper surface of the tightly stretched 

 rubber sheet and chilled until it is brittle. The paraffin coat is 

 best applied by pouring the melted paraffin very rapidly and 

 freely over the rubber sheet, which has previously been warmed 

 practically to the melting-point of the paraffin. The hot paraffin 

 is allowed to drain from the rubber. A little experience soon 



