ULTIMATE FORCES 379 



The wonder is, not that it was able to lift mountains, but that it has not 

 long ago crushed out all differences of density. The application of this 

 force to any part of a reservoir of magma would endue the latter with 

 power to rend anything else. The only thing capable of preventing it 

 would be the same pressure on the rocks restraining the reservoir and in 

 a state of equilibrium. When once this adjustment was disturbed, how- 

 ever, the consequences in intrusion and folding were almost inevitable. 

 An important factor should be pointed out — that is, that the force of 

 gravity was not exhausted by any one sinking of the ocean-floor. It 

 remained just the same, ready for action again if there was release around 

 the margin of the sea. 



A review of Appalachian structure brings out some highly interesting 

 coincidences. The gradual increase of oscillations through the Paleozoic, 

 and the establishment of scanty areas of folding in the Ordovician and 

 of greater ones in the Devonian, point to the gradual upsetting of crustal 

 balance. This was consummated at some time in the Pennsylvanian by 

 formation of the great overthrusts of the mountain border zone, which 

 may well represent the outthrust of the domed sea-floor. Its collapse 

 followed in the Permian, with intrusions and folding. The great shear 

 planes of the first overthrusts were themselves folded and faulted, in 

 many places to as great a degree as the strata themselves. In the later 

 stages the pressure of the folding and the heat and pressure of the batlio- 

 liths metamorphosed the various rocks. 



In short, the kind and sequence of events in Appalachian deformation 

 accord more than usually well with those deduced from the density dif- 

 ferences. The history of the Appalachians is similar to that of other 

 mountain systems in the increase of folding and oscillation until the 

 climax was reached. "While all ranges do not have the same origin or 

 history, the general agreement is worthy of note. It is also of interest 

 to point out that the early overthrusts of the Appalachians correspond 

 well with the faults of the later mountain systems, the Appalachian faults 

 being more deeply eroded while the others still show their surficial aspect. 



In summing up, the theory of batholithic intrusion seeks to explain 

 the formation of the folded Appalachians by pressure from intrusions 

 of magma. These furnished the heat and force required, were of ade- 

 quate power and bulk, and accord remarkably with the varied phenomena 

 of the system, both in space and in kind. No other theory even ap- 

 proaches an adequate explanation. This theory relies only on natural 

 processes Avhose effects are open to observation, and it calls for few con- 

 ditions that are not known to have existed and on none that are unrea- 

 sonable. The basal facts of the theory have been gathered and verified 



