434 TERENCE T. QUIRKE 



position amounting to 90° in extreme cases. Such a combination 

 is illustrated in the breaking of one wooden piece (Fig. 7C) and 

 in the rupture of the soap and paraffin (Figs. 2, 3, 4, and 5). 



Comparisons between geological thrust deformations and members 

 ruptured by rotational stress. — In cases where rock members are 

 subjected to both tension and shear we expect failure to be due to 

 that which the rock is least able to withstand, that is, a combina- 

 tion of both shear and tension. Rocks are strong to resist com- 

 pression, and failure of rock members by straight compression 

 needs little consideration when either great shear or great tension 

 is involved. 



Comparisons between the rupture of these members which 

 fail by combined compression and bending, the rupture of short 

 blocks which fail by pure compression, and the geological relations 

 of overthrust and reverse faults seem to suggest that many thrust 

 faults may have resulted from the yielding of flexed members 

 which have failed along planes of shear and tension, members of 

 the class of long columns. 



Thus it appears that some members under terrestrial compres- 

 sion are curved sheets or strata-plates, which are thin in compari- 

 son with their width and length; being subjected to an unequally 

 distributed rotational stress they break normally along shear 

 planes of low angles at depths which steepen to high angles due to 

 tension near the surface. 



KINETICS OF THRUST FAULTING 



Analysis of the conditions following rupture and preceding dis- 

 placement. — Some rotational force deforms the terrain by flexure. 

 If the folding is sharp no fold can transmit thrust because the fold 

 must fail at the crown of the arch due to locaUzed tension. Rock 

 is incompetent to carry tensional stresses, and therefore a high 

 arch fails under lateral compression. If the arch fails, no appreci- 

 able thrust can be transmitted until folding has reached an isoclinal 

 condition, involving great shortening, great thickening, and thereby 

 great strengthening of the member.^ This case is extreme and 

 must be rare. More commonly the folds are so low that thrust 



' Cf. Quirke, op. cit., p. 72. 



