THE PROBLEMS OF GEOLOGY 607 



tion, and interpret these facts in the hght 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 jaspihte 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 profound 

 folding without rupture, although often there is superimposed upon 

 the flexures more recent fracture deformation. Physical experiments 

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

 beyond the limit of elasticity under the conditions of the earth's sur- 

 face, that 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 

 are within the limits of the force of cohesion. This naturally led 

 to the suggestion of a deep-seated zone of rock-flowage, in opposi- 

 tion to a zone near the surface, that of fracture. At the time this 

 conclusion was reached, no experiments had been made actually 

 showing the deformation of rocks under the conditions of the deep- 

 seated zone, but since that time Adams and Nicolson have deformed 



