186 GEOLOGY OF THE COMSTOCK LODE. 



action seems to have been distributed in part by a forking of the fissure, 

 and in part by the formation of east-and-west cracks. 



Coefficientof friction of rocks involved in the fault. Thc rOcks inVolvcd in the faulting 



action on the Siitro section are diorite, diabase, hornblende-andesite, and 

 augite-andesite. They must all have sensibl}' equal coefficients of friction, 

 for the curve in the western diorite is apparently continuous with that of 

 the other rocks which lie east of the vein, and thei'e is no evidence of dis- 

 continuity in the eastern curve as it passes the contacts. All the east rocks, 

 too, appear to divide into plates of the same thickness, while the diorite has 

 split into sheets of less than half that of the others. 



Rulesapplicabletoprospectingin uneroded districts, It IS, of COUrSe, mOSt UuHkely that 



the CoMSTOCK is the only vein in which the deposition of ore is recent, and 

 has been accompanied by faulting, and some conclusions as to the occur- 

 rence of veins in such cases may be welcome to some of the readers of this 

 paper. 



In a locality modified by faulting action vinder lateral pressure, the fact 

 will appear in the parallelism of the exposed edges and faces of rock-sheets. 



If erosion has not seriously modified the surface resulting from the 

 faulting action, the logarithmic curve will be recognizable to the observer 

 looking in the direction of the strike. 



The main cropping of the vein is to be sought at the point of inflection 

 of the curve, which will be found nearly or exactly midway between the 

 top and bottom of the hillside. One or more secondary vein croppings 

 should be looked for below the main cropping, and these, so far as yield is 

 concerned (but not in regard to location of claim), may prove more impor- 

 tant than the main cropping. 



The dip of the vein will be to the same quarter as the slope of the sur- 

 face, but, of course, greater in amount. The flatter the surface curve, the 

 smaller the angle of dip will be. The mean strike will be nearly or quite 

 at right angles to the direction of the spurs and ravines of the faulted area. 



If besides the movement of one or other wall in the azimuth of the 

 dip, there has been a dislocation in the direction of the strike, chimneys will 

 open, all of them on the same side of the diff"erent ravines. Surface evi- 



