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brecciating layer at first into rhombs bounded by two faults and 

 two bedding planes, and afterward, under increasing pressure, into 

 triangular forms bounded by two faults and one bedding plane. 

 From brittle, thin-layered rocks under slight stresses quadrangular 

 and subquadrangular small fragments are derived. The sharpest 

 edges are found in rocks of conchoidal fracture, minutely faulted 

 into triangular fragments. 



The matrix when contemporaneous is supplied by the material 

 of the weaker beds involved and by wear and tear of the stronger 

 beds. It graduates from powder and angular sand, the product 

 of attrition, to chinkstone, approaching the size of the smaller 

 fragments. When the interstices are left at time of breccia tion 

 more or less unfilled, as is likely to be the case, a veinstone matrix 

 of travertine, of jasperoid, or of iron compounds is often deposited 

 later by ground-water. Such a matrix weighs against any ori- 

 gin, e.g., subaqueous glide, which is not likely to leave unfilled 

 interstices. The significance of both veinstone and attrition 

 matrices lies also in the proof they offer that sedimentary deposits 

 had no access to the zone of brecciation. 



The material of tectonic breccias, with the exception of the 

 matrix of chemical deposit, derives from the geologic formations 

 of the beds involved. Fragments of beds belonging stratigraphi- 

 cally below the base of the breccia cannot be included in it. On the 

 other hand, in subaqueous or subaerial breccias fragments deriving, 

 for example, from cliffs of Archean rock may be deposited as Devo- 

 nian breccia on Devonian lands or in Devonian seas. But while in 

 subaerial and subaqueous breccias the matrix is never older than 

 the fragments, in tectonic breccias an older and weaker terrane 

 may supply the attrition matrix, in which the fragments of a younger 

 superjacent stratum are imbedded. Where beds of different 

 rocks are involved zonal arrangement is sometimes traceable, 

 which at once excludes the breccia from many varieties of sub- 

 aqueous and subaerial origin. A tectonic breccia does not rest, 

 like subaerial breccias, upon an erosion surface. It cannot graduate 

 upward into strata inclosing sporadic fragments. An important 

 diagnostic may be found in undisturbed areas, perhaps of very 

 large size, which have transmitted the strain instead of yielding 

 to it by fragmentation. 



