184 STUDIES FOR STUDENTS 



settling seaward of the fresh-formed Devonian sediments upon the 

 sloping flanks of the Island of Wisconsin, and a slipping of the 

 strata upon one another causing crumpling, buckling, and breccia- 

 tion. The same cause is assigned by Hershey 1 for a very local 

 breccia near Galena, Missouri. Minor causes of subaqueous glides 

 are erosion and undercut of submarine banks by springs and 

 currents. 



The characteristics of glide breccias are due to the deformation, 

 to the shear and crush of the gravitative movement, and not to its 

 precipitory cause. Hence all the varieties mentioned are alike in 

 structure and have a close resemblance to endolithic breccias 

 caused by deformation. 



The fragments are contributed by any layers hard enough to 

 suffer fracture. They may be sharply angular or somewhat worn 

 by mutual attrition. They may be apposed in crackle and mosaic 

 breccias, or disposed in rubble, according to the amount of move- 

 ment. Fragments may show by their relative positions the initial 

 attitude of the layer before fragmentation. These breccias are 

 likely to graduate into folded structures with parallel and thick- 

 ened axes and common dips, and the fragments of beds bent before 

 breaking may show flexures and contorted laminae. A zonal 

 arrangement is to be looked for where strata differing lithologically 

 and of a considerable thickness are involved. 



The matrix is supplied by the least indurated or most readily 

 comminuted beds, especially by the surface sediments as yet uncon- 

 solidated, and by any bottom layer which by its plasticity deter- 

 mines the base plane of the glide. Thus shales furnish matrix to 

 fragments broken from brittle limestones. Matrices evidently 

 pasty and fragments somewhat plastic at time of brecciation point 

 to subaqueous brecciation either by glide or by wave-action, and 

 the former alternative is to be chosen when there are proofs of 

 folding before fracture. This test applies only when brecciation 

 in the zone of flow and fracture is precluded. The relative amount 

 of matrix and fragments is determined by the volumes of strong 

 rock and weak rock involved and by the violence of the movement. 

 Even a breccia of sporadic fragments may result. 



1 O. H. Hershey, "A Devonian Limestone Breccia in Southwestern Missouri," 

 Science, N.S., I, 676-78. 



