180 GEOLOGY OF THE COMSTOCK LODE. 



come under the conditions which have been explained in the preceding 

 portion of this chapter, and, as a matter of fact, if the Sutro Tunnel sec- 

 tion be taken as a representative one, it is easy to find a logarithmic curve 

 which shows a close coincidence with the surface. The eastern and western 

 branches of the curve referred to the fault line, and a perpendicular to the 

 fault line at the cropping of the vein are, respectively, 



?/, = 147(t" (1.00161— •'-l)—.ri tan 44° 27', 



?/, = 1470«- (1 — 1.00298 ^')+.r, tan 44° 27'. 



Knowing these values, the experiment on slips of paper can lie modi- 

 fied to obtain a corresponding result. The only change needful is to pile 

 the slips in such a way that their ends instead of falling in a vertical plane 

 will lie in a plane forming an angle of 45° 33' with the table. The result 

 is a curve, which, when plotted on the assumption of a suitable thickness of 

 the sheets, is indistinguishable from that of one or other of the above equa- 

 tions. Precisely as in the former experiment, too, the position of the asymp- 

 tote precludes the supposition that the ctirve is hyperbolic. There is, 

 therefore, very strong reason to believe that the Siitro section surface line 

 is composed of two logarithmic curves, and no reason known to me to sup- 

 pose that it is not. 



Atlas-])late VII. shows the surveyed surface line of the Sidro section 

 plotted from the contour map, and in the same figure the curve plotted 

 from the equations given above. The same plate also .shows the curves 

 represented by the equations plotted by themselves with their axes and 

 asymptotes, and the curve obtained from experiment. By comparing the 

 surveyed line with the surface maps, it will appear that its deviations from 

 the curve given by the equations are the evident results of plainly limited 

 erosion, the section crossing two considerable ravines in the east country, 

 and passing along the flank of another in the west country. 



Constants. — Tlic dislocatiou measured on the dip of the lode is 2 A, or, for 

 the present case, 2,940 feet. The dip of the lode at this section is 43°, and 

 the dislocation measured vertically is therefore 2,005 feet. The angle S is 

 44° 27' and S is therefore 2° 33', or the original surface sloped contrary to 

 the dip at this angle. The natural unit of the east curve is 2,012 feet, and 



