YIELDING OF THE EARTH'S CRUST BOWIE. 



241 



first step in the latter is to attempt to find a cause of the vertical 

 movement of material of the earth's crust which is local in its char- 

 acter. 



CAUSES TENDING TO THE UPLIFT OF MOUNTAINS AND TO THE 

 BRINGING ABOUT OF SUBSIDENCE. 



By a process of elimination we conclude that the uplift of the 

 mountain system must be due to an actual decrease in the density of 

 the material forming the blocks under the affected area. In order 

 that the isostatic balance may be maintained the density under a 

 mountain area must be less, on an average, than the density which 

 obtains in a block whose surface is at or close to sea level. But the 

 mountain area was once an area of sedimentation as is shown by the 

 character of its rocks. How, then, could the block under this moun- 



12 3 



„, A Sea Level Column 



Ogffi Before Sedimentation 



Level 



Depth of 



Normal volume 

 Normal mass 

 Normal density 



Same After 

 Sedimentation 

 ^3 



Normal volume 



Normal mass 



Density of sediments 

 less than normal :there- 

 fore.avcraqe density 

 of matter he low sedi- 

 ments is greater than 

 before sedirntation. 



SarneAfterUplift 



Volume greater 

 than normal 

 Normal mass 



There fore, density 

 subnormal 



Compensation 



Fig. 6. — Changes in densities in the earth's crust. 



A block, with its surface at sea level, that is in isostatic equilibrium, must have an 

 increase in the density of its original material after heavy sedimentation. It must have 

 a lighter density than normal after its surface has been uplifted. 



tain area have become longer, to the extent say of two miles, and still 

 be in isostatic equilibrium without there having been an actual de- 

 crease in the density of the material of the column? Apparently a 

 decrease in density must have occurred. 



We must now go one step further and inquire as to what could 

 have caused the change in density from normal to subnormal. Let it 

 be assumed that the material of the earth's crust is composed of hori- 

 zontal layers. When an area is undergoing sedimentation the layers 

 of the block under the sediments are being forced down into regions 

 which are undoubtedly hotter than those they occupied prior to 

 the deposition of material at the surface of the block. It is not 

 known definitely or accurately what is the temperature gradient 

 even for the first 5,000 or 10,000 feet of the earth's material for it 

 varies from place to place. As a working hypothesis, however, we 



