340 University of California Publications. [Geology 



A discussion of the porphyry considered as an ore of copper 

 would be far from complete without some reference to what is 

 perhaps its most remarkable feature. This is the contrast whicb 

 its internal structure presents to that of the surrounding rocks. 

 As has been shown in an earlier part of this paper the Carbon- 

 iferous limestones have been folded; but the folds are open ami 

 there is no evidence of intense compression resulting' in plication 

 or shearing' anywhere in the sedimentary terranes. Similarly 

 the monzonite which is the earliest intrusive into these sedimen- 

 tary rocks shows no evidence of deformation or of notable shear- 

 ing. The same is true of the monzonite porphyry. The latest 

 eruptive, a rhyolite lava, is similarly devoid of evidences of com- 

 pressive stresses. All of these rocks are, it is true, traversed by 

 faults but these are not numerous and do not in any case appear 

 to have affected the internal structure of the rocks dislocated. 

 It is probable that they are the local manifestations of general 

 movements of the earth's crust in this part of the Great Basin 

 occurring at a late date in its geological history. The ore bear- 

 ing porphyry on the other hand, although occurring in compara- 

 tively small and definitely limited masses in the midst of these 

 unaffected or but gently folded and broadly faulted terranes, is 

 internally intensely deformed. In the series of long drifts and 

 numerous crosscuts of the Ruth Mine, which the writer examined 

 closely, scarcely a step can be taken without passing slips and 

 shear zones in the body of the porphyry. These have quite va- 

 riable orientations both as regards dip and strike, but in spite 

 of this variation a series of observations in all parts of the mine 

 shows an interesting general tendency. Of some 74 observa- 

 tions for the direction of the dip of these slips and shear zones, 

 only nine were found to bear southerly. In all other cases the 

 direction of dip was in some azimuth to the north of the mag- 

 netic east and west line. That is, the prevailing dip is in the 

 direction of the general dip of the mass of porphyry considered 

 as a whole. ■ The angle of dip ranges from 10° to 75° and the 

 average value of the 74 observations is 40°. Many of these slips 

 and shears intersect one another, the abutment of older slips 

 upon later ones being commonly observed. Many of the shear 

 zones have a black gouge from a few inches up to about a foot 



