258 PHYSICAL GEOLOGY 



Atlantic coast of Canada, where the ancient shore line stands 575 

 feet above tide at St. Johns, Newfoundland, and declines to 250 feet 

 in northern Labrador. The warping of the eastern part of the United 

 States in the Appalachian region has resulted in the high mountains of 

 North Carolina and the lower areas of Pennsylvania. Portions of 

 Sweden are sinking while others are rising. Proofs of such move- 

 ments are not to be seen in folds, since the rocks under these condi- 

 tions are not compressed ; but can be ascertained by careful measure- 

 ments extending over many years, and especially by a study of 

 elevated marine terraces and old land surfaces. The movement may 

 be extremely slow, often being only a few inches a century. 



Zones of Flow and Fracture. — The rock of the earth's crust is 

 much fractured at and near the surface. Such fracturing, however, 

 does not extend indefinitely downward, although at a depth of at 

 least eleven miles * empty cavities may exist in granite, and if the 

 cavities are filled with water, gas, or vapor, they may exist at even 

 greater depths. It is evident, however, that at depths of even eleven 

 miles many rocks less strong than granite will be unable to withstand 

 the enormous weight of the overlying mass and will yield to the pres- 

 sure, not by fracturing but by flowing, after the . manner of wax. 

 The portion of the crust which yields to pressure by fracturing and 

 in which fractures consequently exist is called the zone of fracture ; 

 that portion of the crust below this, in which the rock yields by 

 flowage, is called the zone of flow. In this zone cavities are absent 

 and the particles of rock occupy the minimum space. Since rocks 

 vary greatly in strength, it is evident that the depth to which the 

 zone of fracture extends will vary with the rock, and that conse- 

 quently the upper surface of the zone of flow is very irregular. 

 Between the zone of flow and the zone of fracture is an intermediate 

 zone in which the soft, incompetent beds flow, and the hard or com- 

 petent beds fracture. This is called the zone of flow and fracture. 



Joints 



Attention has been called to the division planes or joints 

 by which all rocks near the earth's surface are more or less broken 

 into angular blocks (p. 25). This structure can be well studied 

 111 almost any quarry or clifF (Figs. I, 5, 85). Joints approach 



1 Adams, F. I).. — An Experimental Contribution to the Question of the Depth of the Zone ol 

 Flow in the Earth's Crust: Jour. Geol., Vol. 20, 1912, p. 97. 



