STUDIES FOR STUDENTS 163 



The residual breccia of the Joplin district of Missouri has been 

 described by Winslow, 1 Smith and Siebenthal, 2 and others. The 

 breccia-producing rocks are Mississippian limestones rich in chert. 

 At the close of the Mississippian the region was uplifted and, under 

 long subaerial erosion, developed a typical Karst topography 

 mantled with angular cherty waste. During early Pennsylvanian 

 times the area was deeply submerged; the Carboniferous sea 

 transgressed, but without either leveling the relief or assembling 

 and rounding the residual cherts. The matrix is largely supplied 

 by the sea-clays of the Cherokee formation. The residual breccia 

 thus formed suffered further changes. Solution of the underlying 

 limestone has caused the breccia to founder, producing complex 

 breccia tion and intermingling blocks of the country rock. Ground- 

 water has also introduced a matrix of lead and zinc ores and jas- 

 peroid silica. 



Talus breccia. — Accumulating at the foot of cliffs, from frag- 

 ments broken off by frost and temperature changes, talus breccia 

 is a rubble of sharp-edged fragments, wedge-shaped in radial section, 

 and quite devoid of bedding. Stratification may be rudely simu- 

 lated, since slabs and other unequiaxed blocks creeping and sliding 

 down the slope come to rest with their longer axes parallel with 

 the surface. Owing to greater inertia, the larger blocks may 

 gather at the base. Small fragments prevail, but blocks of some 

 considerable size may be supplied by local sapping. The matrix 

 (the finer material derived from the weathering of the cliff and of 

 the talus) is interstitial and lithologically identical with the frag- 

 ments. Some of the matrix, however, may be foreign — dust and 

 sand brought in by wind, humus of a soil cover, and travertine 

 deposited by springs issuing from the cliff's base. 



Talus breccias are local and extremely limited in width. Even 

 under an arid climate favoring the perpetuation of cliffs, even in 

 the hamada desert, where long lines of marching cliffs form the 

 high risers for the broad steps of rock plateaus, talus accumulations 

 are rapidly consumed by deflation and are but a few rods wide. 

 The material is local and there is no zonal arrangement of fragments 

 of different lithologic kinds. 



1 Arthur Winslow, Missouri Geol. Surv., VII (1894), 464 f. 2 Op. tit., p. 9. 



