EFFECT OF THRUST. 59 



Result of a Thrust. — Collecting the results of the last few paragraphs, it 

 appears that a simple horizontal thrust from the south-southwest will at least 

 tend to produce three distinguishable effects : 1. A cubical compression upon 

 which exactly one-third of the applied force will be expended ; 2. Two ver- 

 tical sets of fissures, of which one will strike to the north-northwest and the 

 other to the east-northeast ; 3. Two sets of inclined fissures striking north- 

 northwest and dipping at 45°, one set to the east-northeast, the other to the 

 west-southwest. On the vertical fissures the northwesterly wall would move 

 in a southwesterly direction relatively to that opposed to it, and the north- 

 easterly wall would have a relative motion towards the southeast. A thrust 

 on a line striking at right angles to the direction assumed above would pro- 

 duce vertical fissures on the same surfaces, but it would also produce relative 

 movements precisely the opposite of those observed. The hypothesis of a 

 thrust in this direction is thus excluded. A thrust acting in any other di- 

 rection would not produce vertical fissures striking either north-northwest 

 or east-northeast, so that all such thrusts are also excluded. Thus, as pre- 

 viously stated, the observed conditions are fulfilled only by a thrust on a 

 line striking north-northeast to south-southwest. 



While this thrust will tend to produce the effects enumerated above, it by 

 no means follows that all of these tendencies should be permanently trace- 

 able at any one spot. Solids such as granite offer great resistance to cubical 

 compression and are almost perfectly elastic to this stress, so that when the 

 pressure on the mass was relieved the fragments must have resumed their 

 original volume. One can expect to find the tendency to cubical compres- 

 sion manifested only in slickensides and similar phenomena. 



Were the granite ideally homogeneous in all its physical properties, were its 

 surface a true plane, and were the thrust distributed with absolute uniformity, 

 then all parts of the mass would reach the elastic limit at the same instant 

 and all the fissures would form simultaneously. Of course, such ideal con- 

 ditions could not actually prevail, and one should thus expect that in some 

 localities one set of fissures would be developed either alone or in a pre- 

 ponderating degree, and that in other localities other fissure systems should 

 predominate. Even in laboratory experiments, on the crushing of blocks of 

 stone or metal, when all possible precautions are adopted to insure uniformity 

 of conditions, the mass often yields only along a single plane. In the Sierra, 

 on the other hand, at least two systems of vertical parallel fissures, as a rule, 

 accompany one another, and they are often closely attended by associated 

 inclined systems, dipping at 45°. 



The vertical fissure systems I have studied with great care, as has been 

 mentioned above, and their strike has been taken at almost innumerable 

 points. I also noted the existence of the inclined fissures dipping at about 

 45° at many points, but, while in the field, I was under the impression that 



