Typical overthrust Faulting. 141 



On the southern side of the range there is a region of disturbed 

 rocks similar to that on the north, but somewhat wider and less 

 minutely i^licated. The structure is well shown in the lower 

 portion of the Nizzenah canyon, whose walls rise from 2,000 to 

 3,000 feet vertically above the river. One excellent example of 

 faulting was observed. A bed of white limestone about 500 feet 

 in thickness, probably a continuation of the one in which the 

 fan structure was observed on the northern side of the range, 

 has been broken across and thrust over upon itself a distance of 

 half a mile. Within this space there appear to be two conform- 

 able beds of limestone in place of one. The diagramatic form 

 in which the fault is displayed on the canyon wall confirms cer- 

 tain theories as to the mechanism of such faults derived from 

 much more obscure phenomena in other regions. Evidently 

 folding, clue to lateral compression, had been only slightly de- 

 veloped when a shearing fracture took place across the rigid bed. 

 The fracture did not extend far on either side of the limestone, 

 but the thin-bedded black shales above and below are intensely 

 plicated, having taken up the lateral compression by folding 

 instead of faulting. Apparently the conditions which determined 

 the formation of a fault rather than a series of folds in the lime- 

 stone were, first, the great difference in rigidity between that bed 

 and the adjacent shales and, second, the absence of a heavy load 

 upon the beds during the compression. 



Nizzenah river, for about seven miles above its confluence 

 with the Chittenah, flows in a narrow canyon with rocky walls 

 from 400 to 500 feet high. For a short distance above the canyon 

 the gravel bluffs are replaced by cliffs of calcareous black shale 

 apparently very recent and only slightly affected by the com- 

 pression which has disturbed the rocks lying on the north. At 

 the upper end of the canyon the black shale contains beds of 

 extremely coarse conglomerate, and is succeeded by black slate 

 and mica schist, the latter containing many small quartz veins. 

 An east-and-west line through the upper part of this canyon ap- 

 pears to be the approximate limit of the little altered rocks 

 forming the northern range. 



Rocks of Copper River Valley. — Several massive dikes intersect 

 the course of the Chittenah a few miles above its junction with 

 the Copper, forming high cliffs, and a number of rocky islands 

 in the river channel. The dikes are composed of a very com- 



20— Nat. Gkog. Mag., vol. IV, 1892. 



