FAULTS. 53 



Throw of the Faults. — To determine the direction and throw of faults in 

 granite is often (or perhaps usually) impossible, and I must confess with 

 mortification that, prior to the present investigation, I have been over vast 

 areas of granite in the Sierra and even studied the fissure systems without 

 perceiving that, in this region, the faults can very frequently be determined 

 to a tenth of an inch or less. The means of determining the faults under 

 discussion depends upon the existence of the white dikes already referred to. 

 These dikes (which are familiar to all visitors of the Yosemite valley) are 

 distributed over the entire area dealt with in this paper, though they are 

 much more common in some localities than in others. They are often ex- 

 tremely persistent and can sometimes be followed for a mile or more without 

 sensible deviation or change of width. They are seldom more than three 

 feet in width and are more often from half an inch to four inches wide. The 

 welding between the dike-rock and the walls is commonly perfect, but the 

 dividing line is as sharp as a pencil-mark. No doubt the distribution and 

 direction of these dikes is systematic, but I have not yet found the key to 

 the system. They seem to stand at all sorts of angles and are quite as often 

 almost horizontal as in any other position. Their intrusion was accompa- 

 nied by faulting, as a matter of course, and this can occasionally be directly 

 observed ; but there is no danger of confounding these movements with those 

 on the fissures which form the subject of this paper, and which intersect the 

 white dikes. The throw of the later faults is determinable where the white 

 dikes are intersected, because the dikes are also faulted, and extremely 

 minute motions are thus revealed. 



The throw of the faults is, as a rule, so small that it might readily be over- 

 looked, as indeed might be inferred from the very great frequency of the 

 fissures. Suppose, for example, that the total faulting of a point on the 

 range is one mile relatively to another point 200 miles to the southeast of it. 

 Then the average faulting is one foot in 200 feet, or three-tenths of an inch 

 in five feet, which, as nearly as I can ascertain, is the average horizontal 

 distance of the faults. The faults observed are of this order, rarely exceed- 

 ing three inches and often sinking to a quarter of an inch or less ; only in 

 the immediate neighborhood of andesitic intrusions have I detected throws 

 amounting to from two to three feet. Slickeusides are sometimes as well 

 developed by the smallest faults as by the largest. 



Large faults, however, are often simulated in this region. One frequently 

 comes upon a vertical wall in massive granite from ten to fifty or more feet 

 in height which shows manifest slickeusides, and a casual observer w^ould be 

 apt to infer that the height of the wall was the throw of the fault. In most 

 of the cases of this kind which I have seen I have been able to demonstrate 

 by the course of white dikes that the real fault did not exceed a few inches. 

 The explanation is that one or other wall of the fissure has been carried 

 away either by ice or by frost. Sometimes this has occurred because one wall 



