CONTORTIONS OF THE STRATA 



41 



of intersecting fractures which are perpendicular to the free sur- 

 faces of the block and take two directions each inclined by half 

 of a right angle to the direction of compression 

 (Fig. 19). This experiment may illustrate the 

 manner in which fractures are produced by 

 the compression within the zone of fracture 

 of the lithosphere, as its core continues to 

 contract. 



To reproduce the conditions within the zone 

 of flow, it will be necessary to load the lateral 

 surfaces of the block instead of leaving them 

 unconstrained as in the above-described ex- 

 periment. The experiment is best devised as 

 in Fig. 20. Here a series of layers having 

 varying degrees of rigidity is prepared from 

 beeswax as a base, either stiffened by ad- 

 mixture of varying proportions of plaster of 

 Paris, or weakened by the use of Venice turpen- 

 tine. Such a series of layers may represent 

 rocks of as widely different characters as lime- 

 stone and shale. The load which is to rep- 

 resent superincumbent rock is supplied in the 

 experiment by a deep layer of shot. 



When compression is applied to the layers 

 from the ends, these normally solid materials, 

 instead of fracturing, are bent into a series 

 of folds. The stiff er, or more competent, layers are found to be 

 less contorted than are the weaker layers, particularly if the 



FIG. 19. Two inter- 

 secting parallel series 

 of fractures produced 

 upon each free sur- 

 face of a prismatic 

 block of stiff molders' 

 wax when broken by 

 compression from the 

 ends (after Daubr6e 

 and Tresca). 



Section, on Una at> 



Section on Une cd 



FIG. 20. Apparatus to illustrate the folding of strata within the zone of flow 



(after Willis). 



