SUMMARY OF THEORIES 361 



for 1,000 or more miles, because the faunas are the same on both sides 

 of the gap. Appalachian faunas, however (and others as well), extend 

 for two or three times that distance with little change, so that mere 

 amount of separation affords not the slightest ground for Wegener's 

 argument. 



The hypothesis apparently must, therefore, fall back on a cause which 

 is of the order of the special convulsion of nature appealed to in the 

 early stages of geologic work. Such a convulsion, which affected a whole 

 hemisphere, could have no probable cause except one of an astronomic 

 nature. A cause of similar nature is mentioned by him in the form of a 

 shifting of the poles. Such a change has long been considered by astron- 

 omers and physicists, with the conclusion that it can not be conceded to 

 have taken place, in view of astronomical data, at a date recent enough 

 even to approach the period required by this hypothesis. 



SUMMARY OF THEORIES 



In summing up the main points of the various theories of mountain- 

 building, it appears that all of them are opposed by one or more of the 

 great geologic facts, while most of them receive support in some direc- 

 tion. All of the theories so far proposed depend mainly on some cosmic 

 factor. The principal factor of this kind is gravitation; others of im- 

 portance are the secular loss of heat, a change in the rate of revolution 

 of the earth, and a shifting of the axes of revolution. Some theories 

 combine several of these factors, and they do so with the more or less 

 plain desire of making up for shortcomings in the quantitative results 

 of any one of these causes acting alone. The principal deficiency is in 

 connection with the horizontal motion so plainly evident in the folded 

 ranges. Another great discrepancy lies between the continuity of the 

 forces and of the results, while a third obstacle is in the friction involved. 



The deficiency of horizontal motion which would follow from the old 

 hypothesis that folding was due to condensation of the earth and radial 

 contraction has long been understood. Numerous calculations have been 

 made as to the amount which would follow from condensation due to 

 loss of heat. These calculations are based on the observed increase of 

 heat downward in the earth and the known conductivities of the su])- 

 stances of the crust and of the atmosphere. There is general agreement 

 that the amount of lateral shortening obtainable in this way was seriously 

 short of the requirements. The earth is no doubt shrinking, and this in 

 time causes lateral compression. The time allowable in the case of the 

 Appalachians is only that of the Paleozoic, for the Precambrian deforma- 

 tion must be considered to have absorbed earlier shrinkao^e. It has been 



