EXPLANATION OF THE MOVEMENT 



293 



above sealevel for the same reason that a cake of ice rises above the level 

 of water in which it floats. 



It remains then to inquire if masses of rock may undergo sufficient 

 change in volume from age to age to account for the demonstrated rise 

 and fall of the surface in mountainous regions. This inquiry involves 

 a great number of questions which can not be answered^ some for which 

 the geologist has a qualitative answer, and a few for which a quantita- 

 tive answer may be found. 



It is well known that rock increases in volume with increase in tem- 

 perature and diminishes with decreasing temperature. The rate of 

 heat transfer through rock is shown by Van Orstrand in the appended 

 notes to be so slow that its effects may lag far behind those of erosion 



Figure 2. — Diagram illustrating Deformation of isogeothermal Planes and their Relation 



to Erosion 



.and deposition. As the Ancestral Eockies were lifted from position a 

 to a^ of figure 2, the isogeothermal planes beneath them were bent upward 

 from position h to &'. As material was eroded from the top a' the earth 

 block continued to rise and maintain isostatic balance, carrying the 

 isogeothermal planes still higher. The lifting of the isogeotherms was 

 opposed by cooling, which tends to lower them. But their movement 

 seems to have been so slow that the mountains were eroded away to c 

 by the time the isogeotherm V was lowered to some such position as c\ 

 .As the earth block continues to shrink, due to continued lowering of 

 isogeotherms, the surface is drawn below sealevel, allowing sediments to 

 accumulate on it. This sinking of the Eocky Mountain region began 

 about the middle of the Cretaceous period. A picture of the later action 

 is given in figure 3. 



