Bulletin of the Geological Society of America . f the mountainshad been forced up by injection of material. In other words, the high-lands are highlands because the material of the mountain block is lessdense than the material of neighboring blocks. The mountains rise EXPLANATION OF THE MOVEMENT 293 above sealevel for the same reason that a cake of ice rises above the levelof water in which it floats. It remains then to inquire if masses of rock may undergo sufficientchange in volume from age to age to account for the demonstrated riseand fall of the surface in mountainous regions. This inquir
Bulletin of the Geological Society of America . f the mountainshad been forced up by injection of material. In other words, the high-lands are highlands because the material of the mountain block is lessdense than the material of neighboring blocks. The mountains rise EXPLANATION OF THE MOVEMENT 293 above sealevel for the same reason that a cake of ice rises above the levelof water in which it floats. It remains then to inquire if masses of rock may undergo sufficientchange in volume from age to age to account for the demonstrated riseand fall of the surface in mountainous regions. This inquiry involvesa great number of questions which can not be answered^ some for whichthe 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 ofheat transfer through rock is shown by Van Orstrand in the appendednotes 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 ato a^ of figure 2, the isogeothermal planes beneath them were bent upwardfrom position h to &. As material was eroded from the top a the earthblock continued to rise and maintain isostatic balance, carrying theisogeothermal planes still higher. The lifting of the isogeotherms wasopposed by cooling, which tends to lower them. But their movementseems to have been so slow that the mountains were eroded away to cby the time the isogeotherm V was lowered to some such position as c\.As the earth block continues to shrink, due to continued lowering ofisogeotherms, the surface is drawn below sealevel, allowing sediments toaccumulate on it. This sinking of the Eocky Mountain region beganabout the middle of the Cretaceous period. A picture of the later actionis given in figure 3. 29
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