74 



G. F. BECKER FINITE STRAIN IN ROCKS. 



in the fact that joints frequently die out; but faulted fissures, even those 

 carrying important ore deposits or considerable dikes, also die out. 

 Nevertheless, the dying out of joints shows that the movements involved 

 must at some points be microscopic, and indeed submicroscopic. 



M Daubree has succeeded in reproducing jointed structure in the 

 most striking manner by pressure on mixtures of beeswax and resin. 

 The following cut is copied from his experimental geology and explains 

 itself: 



Figure 10. — DaUbr&e's Experiment on Crushing. 



Here, as in nature, there are joints which die out, but they are associ- 

 ated with faults of measurable throw. The system of divisions is pre- 

 cisely that deduced for a direct pressure in the earlier portions of this 

 paper from the theory of strain. 



Still another lesson can be learned from this experiment. The sides 

 of the crushed column bulge in such a manner as to show that plastic 

 deformation has taken place as well as rupture. Now since these rup- 

 tures can be conceived only as relative tangential movements pushed to 

 the limit of cohesion, it seems to me clear that the plastic deformation 

 also must consist in relative tangential movement, and. indeed, in the 

 same directions as the joints, but not reaching the limit of cohesion. If 

 one inquires what is the effect of this plastic movement on the structure 

 of the mass, one can only reply that it must be something very analo- 

 gous to schistosity. 



