STUDIES FOR STUDENTS 189 



horizon in the same area. The breccia is endostratic arid often of 

 the crackle or mosaic type, with fragments rotated but slightly 

 in the ledge. 1 



In all tectonic breccias the fragments are left of sharpest edge 

 at time of breaking, and a universal sharp angularity points strongly 

 to a tectonic or to other, endolithic origin. Yet the fragments may 

 be worn bygrinding one upon another in the zone of shear and thus 

 become so completely rounded as to be readily mistaken for the 

 pebbles of a wave-laid conglomerate. Thus are produced the 

 "pseudo-conglomerates" of Van Hise. 2 The rounding of pebbles 

 of a conglomerate, however, is pretty uniform for pebbles of any 

 given size, the smaller being better rounded than the larger. More- 

 over, the conglomerate graduates into finer sedimentary deposits. 

 The rounding of the fragments of a pseudo-conglomerate is fairly 

 uniform for any given portion of the brecciated zone. Tracts 

 where fragments are well rounded regardless of size pass into tracts 

 where all the fragments are angular. 3 Dale 4 has noted that angular 

 pebbles of soluble rock in an insoluble matrix may be later rounded 

 by the dissolving action of acid ground-water. It may be added 

 that by solution under pressure fragments develop salients and 

 re-entrants, wholly different from either fracture planes or round- 

 ing by attrition or solution. Fragments may also show in flexed 

 and contorted laminae evidence of the strain to which their layers 

 were subjected. Such distortion phenomena imply considerable 

 plasticity in the layers, although it was finally exceeded by the 

 strain. Contorted laminae may also be seen in the fragments of 

 breccias originating in subaqueous glides, where the plasticity of 

 the sediment is due to imperfect lithification. 



The form and size of the fragments of tectonic breccias so far 

 as due to fracture depend on the amount of stress, the closeness of 

 joints and bedding planes, and the natural fracture of the rock. 

 Thus in one of the experiments of Willis 5 fault planes divided a 



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



2 Van Hise, op. cit., p. 679. 3 Ibid., p. 680. 



4 T. N. Dale, "Structural Details in the Green Mountain Region," U.S. Geol. 

 Surv., 16th Ann. Rept., Part I, p. 569. 



5 Op. cit., PI. 93. 



