RUPTURE. 49 



guished. Granites, and even conglomerates, often break under pressure 

 in extraordinarily smooth, continuous, plane surfaces. Under tension 

 the rupture of granite would follow an irregular surface of least resist- 

 ance, leaving projecting crystals on each side ; and in conglomerates few 

 pebbles would be broken, nearly every one adhering either to one frag- 

 ment or the other. Stratified rocks under tension would behave much 

 like a wooden bar. Only unusually uniform rocks could give smooth 

 surfaces of rupture under tension. Such surfaces do occur in the case of 

 columnar eruptives, and these columns can be shown to be produced by 

 tension in the cooling mass. Even when tension produces surfaces of 

 rupture which are smooth, they are apt to be curved or broken. In a 

 word, tension tears masses asunder; pressure cuts them to pieces. 



Geological Applications. 



Cases to be considered. — It is probable that pure dilation and pure irro- 

 tational shear are strains of rare occurrence in rock masses. One of these 

 requires two, the other three pairs of forces acting at right angles to one 

 another with identical intensity. Simple pressure, on the other hand, is 

 common, especially where disturbances are not in progress. During 

 orogenic changes inclined pressures must be frequent. The most im- 

 portant stress systems are therefore direct pressures and inclined press- 

 ures. The last includes two cases, in one of which the mass suffering 

 pressure rests upon or against an unyielding support, while in the other 

 the mass rests upon or against materials which yield readily. In the 

 former of these cases the stress system reduces to a simple pressure, com- 

 pounded with a scissive stress ; in the latter to a pressure and a shearing 

 stress. 



In dealing with each strain viscosity and a tendency to How or rupture 

 must be considered, the aim being to relate actual phenomena to their 

 immediate causes and to enable the geologist, in some measure at least, 

 to judge of the local direction of the forces the effects of which he observes. 



When gravity acts noon a mass homogeneous strain is. strictly speak- 

 ing, impossible, excepting within infinitesimal limits of space, each level 

 surface being subjected to greater pressure than the next above it. On 

 the other hand, the forces involved in the deformation and fracture of 

 rocks are very great, except in some extreme instances, such as that of 

 moist clay. For ordinary firm rocks the ultimate strength is such that 

 a column of from one to several thousand feet in height would be needful 

 to produce at its base a pressure sufficient to induce rupture. Conse- 

 quently, in masses of such material from a few score of feet to n lew hun- 

 dred feet in thickness, gravity plays but a small part compared with 



