1 88 STUDIES FOR STUDENTS 



other trace of yielding. In the breccias of the Wapsipinicon stage 

 of the Iowa Devonian, a thick, tough, crystalline-granular coquina 

 is normally broken into large slickensided blocks which retain 

 something of the flexures into which the bed was thrown, while 

 subjacent thinly laminated calcilutites are shattered to a jumble 

 of small fragments. 1 



Shales yield plastically under load, but when near the surface 

 and under sudden stress they easily crush to breccia. Shales pro- 

 mote the fragmentation of inclosing and especially of included beds 

 of other rocks. Thus the Wapsipinicon breccias of Iowa embrace 

 the Independence shale and its associated limestones. In the zone 

 of fracture and flowage, alternate thick layers of brittle and of 

 plastic rock may produce brecciated beds, alternating with folded 

 layers, whose arches may be truncated by the movement of the 

 fragments of the rigid and brecciated beds. 2 



Unlike glide breccias, breccias due to folding are included 

 between strata which have shared the brecciating deformation 

 according to their competency. But since any sort of breccia may 

 be involved along with the associated terranes in a later deformation 

 further proof of origin must be looked for in the remains of initial 

 folded structures in the breccia. A certain continuity may be 

 traced from fragment to fragment, showing clearly that the frag- 

 ments are constituent parts of a flexed and broken layer. The 

 breccia may graduate into folds. Where the breccia involves beds 

 of different kinds of rocks, anticlines and synclines may sometimes 

 be traced in a zonal arrangement of the crushed material. 3 



Crush breccias: The sheet breccias of the Joplin district, 

 Missouri, illustrate how terranes of brittle rock may be brecciated 

 by lateral pressure without any further mass deformation than that 

 exhibited in gentle warpings. Heavy ledges of chert have been 

 thoroughly and finely crushed in places and cemented by a chemical 

 deposit from ground-water. The fragments are of small size and 

 are thus in direct contrast with the residual breccias of a higher 



1 W. H. Norton, Iowa Geol. Surv., IV, 158-61. 



2 C. R. Van Hise, "Principles of North American Pre-Cambrian Geology," U.S. 

 Geol Surv., 16th Ann. Rept., Part I, p. 681. 



3 W. H. Norton, Iowa Geol. Surv., IV, 165. 



