VERTICAL MOVEMENTS. 61 



west-northwest to east-southeast. Such fissures, if they exist in the area under 

 discussion, have been overlooked, but were observed near Mt. Lola. 



What Forces are indicated f- — It is certain, then, that there has been such a 

 shearing stress in action as is shown in figure 5, and the question is. How is 

 it to be accounted fi^r? It must arise in one of two ways. If it is a simple 

 shear and not one feature of a compression, it has been caused by a vertical 

 thrust opposed by a resistance not in a vertical line with it and represented 

 by b or, what amounts to the same thing, it has been opposed by a vertical 

 upward force not in the same vertical with it. These forces with a resist- 

 ance to rotation represented by the couple c d would clearly give the ob- 

 served result. On the other hand, if the shear is one feature of a compres- 

 sion, this must have acted downward at an angle of 45°. 



Before making a choice between these hypotheses it is necessary to re- 

 member that the vertical and the horizontal movements took place simul- 

 taneously. This is proved by the scratches on the faulted surfaces, which 

 are sensibly straight lines inclined to the horizon. They show that points 

 originally in contact have moved apart in straight lines, or that the motions 

 in a vertical and horizontal direction have been simultaneous. Hence, too, 

 the forces acting on any particle were reducible to a single resultant, P, 

 which acted either downwards from the south-southwest or upwards from the 

 opposite point of the compass. 



Any force P is resoluble into three components at right angles to one 

 another, say X, Y and Z, and one may, if he pleases, take Xand J^as hori- 

 zontal and Z as vertical. The horizontal components may also be combined 

 to a single horizontal resultant, say E, and when the eflfect of E has been 

 determined it only remains to determine the effect of Z. 



A vertical Couple. — Now in the preceding pages the direction" and eflfect 

 of E have been studied and only the vertical component Z remains. Hence 

 an inclined force producing a compression accompanied by the shear of figure 

 5 is out of the question. Further, if Z were opposed directly by a resistance 

 it would tend to produce four sets of fissures, all cutting the vertical at angles 

 of 45°. Hence these vertical dislocations can be due only to the force Z 

 acting against a resistance which is also vertical, J)ut not in the same right 

 line. This conclusion is of much importance, -as will appear later. 



The couple formed by the vertical force Z and the resistance tended to 

 rotate the mass of the Sierra toward the Pacific ocean. It was opposed by a 

 resistance to rotation so intense that hundreds of square miles of the terri- 

 tory were divided to an unknown depth into horizontal sheets of granite. 

 Such action implies a vast expenditure of energy, and, since the distance 

 through which relative motion took place was very small, the force which 

 acted through the distance was also tremendous. The fact that in spite of 

 this vast force the vertical fissure systems do not on the whole sensibly in- 

 cline to the westward I shall discuss a little later. 



