THE ARGUMENT FROM EXAMPLE 369 



the greatest northwestward advance of the great salients of North Caro- 

 lina, Pennsylvania, and Vermont, and the greatest development of meta- 

 morphism. The chief exceptions in Appalachian structures, the fan 

 structures, are in complete accord with the theory, but are not otherwise 

 explainable. If the theory is true, then the area between two groups of 

 batholiths should be compressed from both sides and should be raised 

 less than the masses over the batholiths. This is the relation observed 

 in all determinable cases known to the author. 



In the eastward representative of the system in Great Britain the post- 

 Carboniferous granites and the disturbances caused by them have long 

 been known. 



It seems impossible that such agreement in relations should be mere 

 coincidence. No other theory of mountain pressure even begins to ac- 

 count for the systematic grouping of the Appalachian structures, and 

 many of the discrepancies of other theories are plain, in view of what 

 has already been stated. That batholiths are an adequate cause, how- 

 ever, is indicated as probable by the heat and pressure which they possess. 

 Other intrusive masses have deformed large sections of the earth's crust ; 

 great batholiths were intruded at the time of the Appalachian folding 

 and did distend the crust; and the areas of their principal intrusion 

 accord with the areas of maximum thrust, metamorphism, and general 

 advance of the crust in the salients. Since, therefore, this theory is in 

 general possible and involves only known principles ; since it satisfies the 

 probabilities as to amount and character of force ; since similar activities 

 have produced analogous results; and since the visible intrusions are in 

 perfect accord with the very diverse requirements, of Appalachian struc- 

 tures, it seems a well founded conclusion that the theory is in the main 

 sound. There is no call on unknown forces or unknown conditions in 

 the earth's crust — only those which are known and can be verified. 



Method of Attaining horizontal Motion 



fluid condition 



The chief difficulty to be solved by this theory of mountain-building 

 is, as in the case of other theories, that of producing horizontal motion. 

 This theory has a decided advantage, however, in that the hydrostatic 

 element is involved in its action. Theoretically magmas are fluid or 

 substantially so, and the local evidence of their condition which we can 

 now see in the rocks indicates fluidity. It scarcely needs mentioning that 

 the intrusive material of dikes, sills, etcetera, was fluid. The evidence 

 is equally clear for the batholiths, thcu^^h hardly as striking. The mar- 



