DEFORM A TION OF ROCKS 6l I 



the mass folded. If the folds are nearly horizontal — that is, 

 if the force was mainly in a single direction — the strike joints 

 may be strongl}' developed and few dip joints produced. If, on 

 the other hand, the folds are important in both directions, the 

 strike and dip joints will both be important. 



Fig. 13. — Radial cracks due to tension in sharply flexed stratum. 



Daubree has shown that if a brittle plate fractures when it is 

 subjected to torsion beyond the limit of elasticity, a double set 

 of parallel fractures nearly at right angles to each other are pro- 

 duced.' The forces which produce complex folding deform the 

 strata, where not too deeply buried, in two rectangular directions 

 by tensile stress ; or, in other words, they are subject to torsion. 

 It therefore appears that Daubree's explanation of joints by tor- 

 sion is but another statement of the production of joints by 

 complex folding normal to the two principal directions of ten- 

 sile stress. 



A question for investigation is the extent of the area over 

 which joints and faults run in rectangular directions. In the 

 case of a large and strongly pitching fold, the force of torsion 

 mav produce rupture in different directions upon different parts 

 of the folds. Upon the flank of the pitching fold, at a proper 

 point, strike joints and dip joints may be formed, striking half- 

 way between the ordinar}^ strike joints and faults, and dip joints 

 and faults at the crests and troughs, and between the two there 

 would be all gradations. 



•Geologic expdrimentale. par A. Dauhree, pp. 306-314, Paris, 1879. 



