540 GEOLOGY 



tion, it was plainly necessary to know the principles of deformation 

 of small masses, such as can be handled in the laboratory. Unfor- 

 tunately it was found that this part of the subject of physics is in 

 a very imperfect condition. No systematic statement is available as 

 to the manner in which different substances behave under varying 

 conditions of stress. While studies have been made of the deforma- 

 tion of iron under a moderate range of conditions, comparatively 

 little has been done concerning brittle bodies such as constitute the 

 rocks. Exact knowledge is needed as to the behavior of rocks under 

 the most extreme variations of stress, temperature, amount of water, 

 and other conditions. But while it is highly desirable to have this 

 knowledge, the geologist cannot wait until it is available. The only 

 practicable course is to study closely the phenomena of rock deforma- 

 tion, and interpret these facts in the light of the physical and chemical 

 knowledge available. 



A broad study of the phenomena of deformation by various men 

 showed two classes of very diverse phenomena. In some areas the 

 prominent deformations of the rocks are those of fractures, such as 

 joints, faults, brecciations, etc. In other places the deformations are 

 mainly those of flexure. For instance, in some places one finds that 

 brittle rocks, such as jaspilite and quartzite, are deformed almost 

 wholly by numerous fractures, and in other places have been bent 

 within their own radius, or even minutely and extremely crenulated 

 with no sign of fracture. A close study of the geological conditions 

 under which these two classes of deformation occurred shows that 

 the more modern rocks, which have at no time been very deeply 

 buried, are those which are most likely to exhibit only the effects 

 of rupture; whereas the ancient rocks, and especially those which 

 have been deeply buried, are likely to show the evidence of pro- 

 found folding without rupture, although often there is superimposed 

 upon the flexures more recent fracture deformation. Physical experi- 

 ments had shown that, when a brittle substance like a rock is 

 stressed beyond the limit of elasticity under the conditions of the 

 earth's surface, cohesion is overcome, and rupture takes place. This 

 fact correlated with the general observation of rupture in recent 

 rocks and those deformed near the surface, led to the conclusion 

 that normally the deformation of the outer part of the earth is by 

 fracture. 



After this conclusion was reached, it was a natural step to the 

 conclusion that at a very moderate depth below the surface of the 

 earth the superincumbent pressure is greater than the strength 

 of any rock, and that, if openings could be supposed to exist, they 

 would be closed by pressure; in other words, that the pressure due to 

 the force of gravitation is sufficiently great, so that the molecules 

 of the rocks are held within the limits of molecular attraction or 



