60 



G. F. BECKER — STRUCTURE OF THE SIERRA NEVADA. 



they were mere local irregularities, and I failed to take systematically miuute 

 notes of their strike. In some instances, at all events, the strike is very 

 nearly north-northwest, and I am not aware of any exceptions. The in- 

 clined fissures are much less frequent than the vertical ones. 



Conclusions as to horizontal Thrust. — On the whole, then, the vertical fis- 

 sures, together with the character of the horizontal movements upon them 

 and with the evident presence of compressive action, necessarily lead to the 

 hypothesis of a thrust acting on a south-southwest to north-northeast line. 

 This hypothesis also compels the inference that there must have been at least 

 a tendency to the formation of two more sets of fissures, dipping at angles of 

 45°, and corresponding fissures have been observed. Thus the facts and 

 hypothesis correspond, so far as is known, while further observations on the 

 strike of the inclined fissures and on the relative movements of the walls of 

 these fissures are requisite. 



YERTICAL MOVEMENTS. 



Resolidion of Forces. — Turning now to the vertical movements, it will be 

 remembered that on the vertical fissure system the northwesterly and north- 

 easterly walls were found to have risen relatively to those opposed to them. 

 If, then, one supposes a vertical section on a strike diagonally across the 

 vertical prisms, or on a north-northeast to south-southwest plane, there will 

 appear on this section parallel vertical lines, representing the corners of the 

 prisms, as shown in figure 5. 



-'— i-f 



I I I I ^ I 



FiGUKE 5— Sectional elevation of Prism. 



Along these lines relative motion has taken place, as indicated by the 

 arrows a b. These arrows show the relative direction of the forces, and 

 may also represent their intensity. Unless they were balanced by a second 

 couple they would produce a rotation instead of strain ; and since the fault- 

 ing is evidence of strain, there must have been a couple, c d, of equal inten- 

 sity and opposed direction. The couple c d would tend to divide the mass 

 into horizontal sheets, and the presence of horizontal partings in many parts 

 of the area has been referred to. The vertical couple would tend either to 

 produce the faulting actually observed on already existing fissures, or to 

 produce fissures striking at right angles to the plane of the diagram ; i. e., 



