1 86 STUDIES FOR STUDENTS 



and produced by lateral or vertical pressure or by tension; (2) 

 expansion breccia (caused by increase of volume due to chemical 

 change); (3) founder breccia (ablation, solution breccia), due to 

 the foundering of strata, usually because of the ablation by solution 

 of the supporting beds. 



Tectonic breccia. — Three varieties are discriminated: fault 

 breccia, fold breccia, and crush breccia. In the latter, brecciation 

 is accomplished without either faulting or folding except so far as the 

 rupture planes of the breccia may be considered as minute faults. 



Fault breccia : Here fragmentation is due either to friction along 

 the fault plane or to distributive ruptures associated with the major 

 fault, and due to shearing stresses. In stratified rock, fault breccias 

 associated with both normal and reversed faults are easily recog- 

 nized, since the zone of brecciation crosses the planes of bedding. 

 Friction breccias along bedding faults are more difficult to dis- 

 tinguish. Here one must seek for proofs of lateral displacement 

 in slickensides with polish, scorings, and seams of "gouge" (clays 

 formed by grinding). Local breccias of this variety have been 

 identified by Ransome 1 in the Rico Mountains of Colorado. 



Complex brecciation is not uncommon, since repeated move- 

 ments along the fault plane shatter and drag a breccia already 

 formed and firmly cemented with perhaps vein stuff and ore. 

 Breccia zones running parallel with the main fault may show but 

 slight displacement. Thus in the San Francisco district of Utah 

 such breccia zones in brittle quartzite pass downward into mono- 

 clinal folds in shale. 2 The rocks on opposite sides of a fault plane 

 may be differently affected — granite, for example, may be sheared 

 and the limestone opposite brecciated. 3 Fault breccias often 

 form aquifers for mineralized waters which deposit matrices of 

 ore and gangue. Many breccias of this class have been brought 

 to notice because of their economic importance. 



1 F. L. Ransome, "Ore Deposits of the Rico Mountains, Colorado," U.S. Geol. 

 Surv., 22d Ann. Rept., Part II, p. 297. 



2 B. S. Butler, "Geology and Ore Deposits of the San Francisco, etc., Districts, 

 Utah," U.S. Geol. Surv., Prof. Paper 80, p. 72. 



3 W. H. Emmons and F. C. Calkins, "Geology and Ore Deposits of the Phillips- 

 burg Quadrangle, Montana," U.S. Geol. Surv., Prof. Paper 75, p. 151. 



