MOUNTAIN FORMATION BY VERTICAL VS. HORIZONTAL FORCES 283 



the cause of the uplift by vertical movements. The speaker discussed 

 this subject in two papers which have recently appeared. 8 



Of course, it is not known what is the real cause of a change of density 

 in a column under a sedimentary area, but by a process of elimination we 

 have left, as a possible cause, the great change in temperature which must 

 take place as the sediments are increased in thickness. We have evidence 

 that areas have had as much as 30,000 or 40,000 feet of sediments laid 

 down on them in one geological age. Let us assume a thickness of 30,000 

 feet, or about six miles. All of the sediments undoubtedly were deposited 

 at an elevation very little different from sealevel. This means that, as 

 the sediments were laid on, the block of the earth's crust under them sank 

 deeper into the earth. Most of the material of the block was lowered 

 approximately to the extent of the depth of the sediments over it, and 

 consequently the material lowered would be raised in temperature by an 

 amount equal to the difference in temperature for the change in depth. It 

 is not known what the temparture gradient is below the outer 7,000 feet 

 of the earth's material, but if we have for the first 60 miles or so of the 

 earth a temperature gradient equal to what we find near the surface, the 

 difference in temperature for a six-mile change in depth would be ap- 

 proximately 300 degrees centigrade. 



If no increase in temperature occurred in the material of the block 

 until after sedimentation had ceased, then the geoisotherms would have 

 been depressed to the extent that the material in the block went down. 

 The geoisotherms would eventually rise to their normal positions through 

 the depressed block and the change in temperature would undoubtedly 

 have some effect, physical or chemical, or a combination of both, on the 

 material, which might expand it sufficiently to cause uplift at the surface, 

 thus transforming a surface of sedimentation approximately at sealevel 

 into an area of uplift thousands of feet in elevation. It is probable that 

 there is a tendency for the geoisotherms to maintain their normal depths 

 below sealevel during the process of erosion and subsidence, but when we 

 consider that the area of erosion may be 100 or 200 miles in width and 

 many miles in length, we are inclined to believe that there is a consider- 

 able lag in temperature of the subsiding column. It is true, of course, 

 that the material below the bottom of the column and at its sides will 

 tend to give off its heat to the colder subsiding material, but this process 

 will tend to cool the sides and bottom material and thus retard the pas- 

 sage of heat from the bottom and sides to the subsiding column. The 



8 The relation of isostasy to uplift and subsidence. American Journal of Science, vol. 

 2, July, 1921. 



Some geological conclusions from geodetic data. Proceedings of National Academy 

 of Sciences, vol. 7, no. 1, January, 1921, pp. 23-28. 



