294 W. T. LEE BUILDING OF SOUTHERN ROCKY MOUNTAINS 



The surface a is depressed with added load to a', and the isogeothermal 

 plane h sinks toward &'. The isogeotherm tends to rise, but its upward 

 movement is slow because of low conductivity of rock. When expansion 

 under rising temperature reduces the density to that of the neighboring 

 rocks, subsidence ceases, but the isogeotherm h' has not yet returned to 

 position 1). As rise of temperature proceeds, the density of the material 

 in the column decreases because of expansion, and the surface rises to 

 position c. This raised part is eroded, and the column, now lighter than 



Figure 3. — Relation of Isogcothenns to a -siiihiitfj Earth Block 



the surrounding material because of decreased density and also because 

 of the removal of surface material by erosion, tends to rise or "fioat'^ 

 upward, as explained below, while material from below is forced into the 

 abandoned space. As the column rises, the isogeothermal surface b is 

 carried up to 6", a position higher than is normal. The tendency of the 

 isogeotherm is now downward, but the lag due to low conductivity allows 



