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



Orientation of the Fissures. — I have shown that the existence of the verti- 

 cal fissure systems attended by evidences of compression leads inevitably to 

 the theory of a horizontal thrust, which is further confirmed by the occur- 

 rence of relatively rare fissures dipping at angles of 45°. Under appropriate 

 limitations it should be possible to invert the argument and say that a hori- 

 zontal thrust would produce the four fissure systems observed. But this 

 is not immediately possible. Why should the vertical prisms not be re- 

 placed by others the axes of which would be perpendicular to the line of 

 force, like those observed, but inclined to the horizon instead of vertical, 

 dipping say to the east-northeast ? Level surfaces cut the actual prisms in 

 rectangles ; they would cut the hypothetical prisms in rhombs. 



Imagine a large cubical portion of the earth's mass with one face at the 

 earth's surface subjected to a horizontal thrust evenly distributed over one 

 of its vertical surfaces. This mass will resist distortion on account of its 

 elasticity, but also because it is surrounded by other masses, and, further, it 

 will, because of its weight, oppose any distortion which tends to raise the 

 center of inertia. Now the horizontal thrust diminishes the volume and 

 thus tends to lessen the opposition which surrounding masses oflfer to its dis- 

 tortion, but cubical compression of course leaves the weight wholly unaf- 

 fected. Moreover, surrounding masses act only superficially on the large 

 cube, while gravity is a " bodily " force acting on every particle alike. 

 Hence when the mass is large, say of a volume of a thousand cubic miles, 

 one would expect to find gravity a much more powerful obstacle to deforma- 

 tion than lateral confinement. If so, the mass would first yield in such a 

 manner as not considerably to raise the center of inertia. Now the only 

 way in which this could be accomplished is by shearing in a horizontal 

 plane, which would tend to form vertical columns. This process, if carried 

 out alone, would not raise the center of inertia at all. 



Comparing this reasonable deduction with the facts, it seems to me that 

 one may conclude with certainty that the vertical fissure systems were 

 formed in that position and because gravity caused a greater resistance to 

 fracture in every other direction than in this. This theory also explains 

 the comparative rarity of the diagonal fissure system caused by the hori- 

 zontal thrust. 



The same theory has a beariug on the fissure systems formed by the verti- 

 cal thrust. In this case horizontal partings could be produced without 

 raising the sheets of granite, while the vertical fissuring and faulting by the 

 vertical thrust component involved either the raising of a sheet against 

 gravity or a downward movement into the underlying mass. When the 

 vertical fissures and the horizontal fissures were once formed, the stress still 

 present would be relieved rather by relative motion on these fissures than 

 by the establishment of new ones. 



