LENGTH AND THROW OF FAULTS 341 



Discussion of certain Features connected with the Faulting 



magnitude 



The small amount of erosion which the basalts have suffered indicates 

 that the lowering of the surface of the basalt layer has not been very 

 great since the ranges were first uplifted. The line along which the fault 

 plane intersected the basalt would, of course, be most rapidly attacked, 

 particularly by landsliding and other forms of undermining, and its 

 fragments would then be the easy prey of agencies of erosion and trans- 

 portation. The top of the Humboldt Lake mountains would be seriously 

 affected by this action. The top of Table mountain, while it may have 

 become narrower from that cause, has hardly become lower. We may 

 use it, therefore, as the upper limit in determining the throw, of the fault. 

 The lower limit is harder to determine, for the lowest parts are covered. 

 Along the surveved section the table is 3,400 feet above the bottom Of 

 the valley between Table mountain and Chocolate butte. This is a 

 minimum value. The Lahontan and other lake beds may be 300 or 400 

 feet or more deep at this point. 



The basalt in the crumbling cliff where the section crosses the Hum- 

 boldt Lake range is 1,500 feet above the patch of basalt at the west base 

 and 2,050 feet above the lowest point of Humboldt valley on the section 

 line. 



The Star Peak range is the highest of the three, but is not capped by 

 basalt. The amount of faulting can not be safely estimated from the 

 highest peaks, as they may have been residual hills on the old topog- 

 raphy several hundred feet high. The ridge line is, on the average, 

 about 3,900 feet above the west valley.* 



The approximate lengths of the ranges measured along the fault mar- 

 gins, which practically means the determinable length of the faults, are : 

 Star peak, 32 miles ; Humboldt lake, 42 miles. The length of the East 

 range is not definitely known, but it is probably longer than the other 

 two ranges put together. 



THE SECONDARY FAULTING 



The breaking up of both the mountain and the valley blocks by minor 

 faulting into secondary blocks has already been described. It appears 

 to be a common condition. Section 3 , plate 16, shows the east slope 

 of the Sierra Nevada just northeast of lake Tahoe. ' This slope represents 

 the eroded scarp of the range. The block is here broken by a fault, with 

 a throw of 2,500 to 3,000 feet. Other examples might be given from the 

 same mountains. 



* Topography of Fortieth Parallel Survey. 



