790 



PRINCIPLES OF STRATIGRAPHY 



Crosby (2) has shown that, if a shock were sent through the strata 

 before this limit was reached, the fracture would be produced in a 

 similar manner. The shock may be produced by the giving way 

 of the first portion of the bed, in which torsion has been carried 

 beyond the resisting strength of the bed, or it may have extraneous 

 causes. 



The slight amount of torsion required in a brittle bed is easily 

 accounted for by differential uplift of the bed. It may be illus- 



FiG. 174. Illustration of Daubree's 

 method of breaking a sheet of 

 glass by torsion to produce inter- 

 secting joints. 



Fig. 175. Arrangement of fractures 

 in glass plate broken by torsion. 

 (Daubree.) 



trated by the torsion produced in a large sheet of stiflf cardboard 

 lifted slightly by one corner. 



Compression joints are produced in the folding of rocks. Simply 

 folded rocks will have joints in two planes at right angles to each 

 other. Joints of this kind are closely related to fissility, which is 

 distinguishable only by the greater number and closer approxima- 

 tion of the shearing planes. "The same compression might produce 

 fissility along one set of shearing planes and joints along another." 

 (Van Hise-23 : d//. ) 



Minor characteristics of joint faces. 



Feather fracture. This, as shown by Woodworth (26), is char- 

 acteristic of the joint surfaces of certain line-grained phyllites or 



