STUDIES FOR STUDENTS 187 



Fold breccia: Under suitable conditions of load and stress beds 

 may fold by means of minor fractures. Cross and parallel fracture 

 planes develop, and under increasing stress the fragments are 

 rotated and displaced. The folded bedding is more or less com- 

 pletely destroyed and there results a mosaic or even a rubble breccia. 

 Where the rock is compressed joints or fissility ruptures are pro- 

 duced. A rising anticline develops radial tension cracks along the 

 convex surface as soon as the deformation passes the limit of 

 elasticity of the rock. In the experiments of Willis 1 the first 

 fractures of the arching strata occurred at points of sharpest curva- 

 ture — along the axial plane at the summit of the anticline and on 

 radial planes of shear at its base. A basal weak stratum, com- 

 pelled to rise beneath a competent stratum as the latter was bent 

 upward, accommodated itself to the change by shear, resulting in 

 complete brecciation . 2 Here the breccia occupied the center and 

 base of the anticline, while the competent upper beds remained at 

 first unbroken and later under increased pressure were ruptured 

 by a stretch thrust fault. 



The degree of folding necessary for brecciation varies with the 

 rigidity of the stratum, with load, and with the amount and rapid- 

 ity of application of the stress. Even limestone and granite under 

 slight load yield plastically and bend to a perceptible degree when 

 the stress is very slowly applied. 3 On the other hand, brittle rock 

 under presumably sudden stresses breaks into breccia when the 

 deformation has not exceeded a gentle warping. 4 Under the same 

 strain and load different rocks fold or break according to their 

 elasticity. Among sedimentaries the most brittle and therefore 

 the most liable to brecciate are cherts, some shales, and calcilu- 

 tites ; among metamorphic rocks, quartzites, graywackes, and rather 

 siliceous slates. A thin layer of chert may be seen broken into 

 a string of angular bits within a layer of limestone which shows no 



bailey Willis, "Mechanics of Appalachian Structure," U.S. Geol. Surv., 13th 

 Ann. Rept., Pis. 75, 76. 



2 Ibid., PL 93. 



s Arthur Winslow, Am. Jour. Sci., 3d ser., XLIII, 133-34; H. F. Bain, Iowa 

 Geol. Surv., VIII, 378. 



4 Smith and Siebenthal, U.S. Geol. Surv., Geol. Atlas, Joplin Folio, p. 9. 



