CHAPTER V 



V 



CONTORTIONS OF THE STRATA WITHIN THE ZONE OF 



FLOW 



The zones of fracture and flow. It is easy to think of the 

 atmosphere and the hydrosphere as each sustaining at any point 

 the load of the superincumbent material. At the sea level the 

 weight of air upon each square inch of surface is about fifteen 

 pounds, whereas upon the floor of the hydrosphere in the more 

 profound deeps the load upon the square inch must be measured 

 in tons. Near the lithosphere surface the rocks support by their 

 strength the load of rock above them, but at greater depths they 

 are unable to do this, for the load bears upon each portion 

 of the rock with a pressure equivalent to the weight of a rock 

 column which extends upward to the surface. The average 

 specific gravity of rock is 2.7, and it is thus easy to calculate the 

 length of the inch square column which has a weight equivalent 

 to the crushing strength of any given rock. At the depth repre- 

 sented by the length of such a column, rocks cannot yield to pres- 

 sure by fracture, for the opening of a crack implies that the rock 

 upon either side is strong enough to prevent the walls from clos- 

 ing. At this depth, rock must therefore yield to pressure not by 

 fracture, as it would at the surface, but by flow after the manner 

 of a liquid ; and so the zone below this critical level is referred to 

 as the zone of flow. 



In contrast, the near-surface zone is called the zone of fracture. 

 But different rocks possess different strengths, and these are 

 subject to modifications from other conditions, such, for example, 

 as the proximity of an uncooled magma. The zone of flow is 

 therefore joined to the zone of fracture, not upon a definite surface, 

 but in an intermediate zone described as the zone of fracture and 

 flow. 



Experiments which illustrate the fracture and flow of solid 

 bodies. A prismatic block prepared from stiff molders'. wax, 

 if crushed between the jaws of a testing machine, yields a system 



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