THE MECHANICAL INTERPRETATION OF JOINTS 721 



lies favorable to the differential movement resulting from the 

 process of torsion. Here, again, the fractures produced on the 

 upper surface extend down to the lower surface and vice versa. 



nntic I in a I 

 bulq e. 



95*4 r - I 



\ -* •„ 5 beneral 



\ 3 •': -> 



easTwarc} dip 



Fig. 7. — A. Map sketch, showing relation of stations on Crooked Creek, Adams 

 County, Ohio, at which joints were measured, to the fault and to other structural 

 features. B. Diagram showing the position of the joints observed at the stations i, 

 2, 3, 4, 5. ^i 52 = shearing joints; ^=tension joints. 



Since the intensity of the compressive stress increases with the 

 thickness of the plate, it is evident that the fan-shaped groups of 

 cracks will form the more freely the thicker the plate is. 



The most striking feature of Daubree's famous experiment, 

 therefore, namely the formation of two systems of fractures inter- 

 secting approximately at right angles, is an accidental result of 

 the exceptional brittleness of glass and the thinness of the plates 



