MOUNTAIN FORMATION BY VERTICAL VS. HORIZONTAL FORCES 285 



miles. This will mean that each particle is brought into a zone which is 

 colder than the one it formerly occupied. If we assume the same tem- 

 perature gradient as before, we shall have a change in temperature of 

 each of the particles of the column under the area of erosion of about 300 

 degrees centigrade. It is hardly probable that the geoisotherms maintain 

 their normal depths below sealevel during the process of uplift. The only 

 way that the material of the block can lose its heat is at the surface and 

 at the sides of the rising block. Undoubtedly, there is some cooling of 

 the material of the rising block as erosion takes place, but it seems prob- 

 able that the geoisotherms are far above their normal positions at the 

 time erosion ceases. When erosion has ceased and there is a period of 

 quiescence, the geoisotherms would, undoubtedly, return to their normal 

 position through loss of heat at the surface and to a certain extent at the 

 sides of the block. During the process of this change of temperature, 

 there would be a thermal contraction of the material of the block which 

 would change its length by approximately 3,500 feet if the cubical coeffi- 

 cient of expansion of the material of the block were 0.000038 per degree 

 centigrade and if the difference in temperature were 300 degrees centi- 

 grade. Whether this is all of the contraction that occurs in a block under 

 an area of erosion after erosion has ceased is, of course, not known, but it 

 would seem to be logical to conclude that contraction is also occurring 

 from some other cause than thermal contraction, which will make the 

 surface of the area sink to sealevel or even below. 



It is probable that contraction of a column that was once under an 

 elevated area continues by loss of heat and some other process during the 

 time that sediments are being placed on the area. It is the speaker's belief 

 that this contraction of a column under an area of former active erosion 

 is a factor in determining the position at which sediment is deposited 

 subsequently. There seems to be a cycle by which we have uplift and 

 erosion in one area with a downwarping and sedimentation in another 

 area not far distant, followed by baseleveling and contraction of the col- 

 umn under the erosion area and an expansion and uplift of the column 

 under the sediment. Areas of erosion and sedimentation have changed 

 places in the past and it would appear possible that this change takes 

 place continuously. It is understood that the Alps have been a region 

 of erosion and of sedimentation several times; also that the Appalachian 

 system has been an area of elevation and an area of sedimentation more 

 than once. It may be that the change of temperature in a block that is 

 sinking under sediments and another block that is rising under erosion 

 may not be the real cause of the expansion in one case and the contrac- 

 tion in another, but at least we have the processes which are competent 



