298 AV. T. LEE BUILDIXG OF SOUTHERX ROCKY MOUNTAINS 



100 X 60 X 5,280 = 1,203.84. In other words, a rise of the surface of 

 1,200 feet may be attributed to expansion of material, leaving the greater 

 part of the known rise to be accounted for in some other way. 



As the surface a of the diagram, figure 3, is depressed 10,000 feet to a', 

 the isogeothermal plane h sinks to h\ where the normal temperature is 

 100 degTees centigrade higher than at h. The isogeotherm tends to rise, 

 •but this rise is so slow, because of the low conductivity in rocks, that 

 reheating takes place after most of the sinking has been accomplished. 

 However, when expansion under rising temperature begins, subsidence 

 ceases; but, because of the 10,000 feet of new material, this balance is 

 reached long before d' rises to h. As the temperature of the block in- 

 creases, the surface a is lifted to position c, the maximum for the 60-mile 

 block being 1,200 feet, and erosion still further lightens the block, allow- 

 ing it to float up so that the isogeothermal j^lane h is raised to h". Ma- 

 terial from the side below the depth of compensation^ enters and forces 

 up the lightened block. The tendency of the isogeotherm is now do'wn- 

 w^ard through the material (but upward relative to any fixed horizontal 

 surface), but the lag in effect due to low conductivity allows considerable 

 rise before cooling increases the density and reverses the movement. 



The experiment with steel tape reported by Van Orstrand has a sig- 

 nificant a]3j)lication to the yielding of solid matter under stress (and 

 perhaps also to the change in density of rock under load). He shows that 

 elongation of the taj^e under a load of less than one-twentieth of the 

 breaking load and the return on relief of stress is many times greater 

 than the change necessary in a column of rock to produce geosynclines 

 and mountains. 



One of the difficulties in explaining the formation of mountains under 

 isostatic adjustment has been the apjoarent necessity of a zone of fiow, 

 while other lines of evidence call for a high degree of rigidity in the rocks. 

 The rearrangement of matter in the steel tape suggests that transfer of 

 rock in a solid state may take place readily enough to meet all require- 

 ments for the building of the Eocky Mountains without recourse to a zone 

 of easily flow^ing rock. 



Although relatively little is known of the behavior of rocks under con- 

 ditions which prevail miles below the surface, other possible cause.s of 

 subsidence of the Cretaceous Basin are elastic deformation of the rocks 

 and actual compression of rock material under load. A measure of the 

 elastic yielding, under the conditions of temperature, rigidity, etcetera, 

 prevailing at the surface has been computed by Walter D. Lambert, of 



' William Bowie : The earth's i-nist and isostasy. Geog. Review, vol. xii, 1022, p. 021. 



