112 IOWA ACADEMY OF SCIENCES. 



the presence of broad terraces at various elevations and other 

 topographic forms indicate plainly the region has been one of 

 recent oscillation and that the terraces mark stages in the 

 cycles when for a considerable period little movement took 

 place. 



GENESIS OF NORMAL COMPOUND, AND NORMAL 

 HORIZONTAL FAULTING. 



BY CHARLES R. KEYES. 



(Abstract ) 



In the mining districts of mountainous regions the ore- 

 hearing belts are quite often coincident with fault planes. These 

 planes are not usually clean-cut, single slipping surfaces, but 

 consist of a number of gliding faces distinct from one another, 

 sometimes branching, sometimes crossing at low angles, and 

 contain in their immediate neighborhood more or less brec- 

 ciated material. This compound character of what we are 

 prone to pass over as single, simple dislocation, is found, 

 ^fter a little careful examination, to prevail in the majority of 

 cases. 



As the slipping commonly occurs in districts in which fold- 

 ing of strata has been more or less intense, it is not frequently 

 taken for granted that the dislocation is of the nature of 

 reverse faulting. This conclusion is apt to be reached when 

 the detailed proofs are obscure, or not clearly made out. 



In many cases, in regions in which the strata have been 

 folded, normal faulting is known to be of frequent occurrence. 

 The illustrations are numerous. The beautiful examples 

 depicted by Spurr in his recent work on the Aspen mining 

 district of Colorado are especially noteworthy as typical 

 developments of the normal compound, or normal horizontal 

 faults. Although not clearly shown in his diagrams, in other 

 localities it is known that the origin of the phenomena is due 

 to a comparatively sudden relaxation of the pressure, allowing 

 the crest of a fold to settle somewhat. When the strata are 

 gradually bowed upward, they do not fracture but flow, as it 

 were, into position; but when the compressing forces are 

 relieved suddenly the layers cannot respond in the same way. 

 They are broken. 



