374 A. KEITH OUTLI^'ES OF APPALACHIAN STRUCTURE 



pulses. When magma was expelled from one region it could move later- 

 ally and raise another region. In this manner we may reasonably explain 

 the persistence of certain areas (probably those of weakness) as domes 

 or ridges and the repetition of movements on them so often shown in 

 the Paleozoic. It should be clearly noted that this process is reasonable,. 

 since it depends on only two factors, fluidity of magmas and horizontal 

 partings at depths in the crust, both of which are known to have existed. 



Magmatic movements also have an immense advantage over isostatie 

 miderflcw in that the moving masses are fluid, and not solid rock with 

 friction and rigidity to be overcome. Independent evidence that mag- 

 mas moved at depths is seen in the fact that batholiths are deep-seated, 

 for they are only exposed by removal of a thick cover of sediment. In 

 fact, if they had been formed near the surface, they would have found 

 that resistance was least to an upAvard movement and they would have 

 exerted little lateral pressure, which is not found to be the case. The 

 corollary to this is the conclusion that they were deep-seated and that, 

 since they did move horizontally for long distances, they found horizontal 

 channels for easier lateral transfer. Under this conception the volcanic 

 plugs which cut the Paleozoic rocks in a few places may rise from the 

 tops of batholiths, boring and pushing their way upward and perhaps 

 lifting the small domes which can not be explained by lateral com- 

 pression. 



The lateral movement of magmas has another close relation to isostasy. 

 As has already been noted, isostasy is a leveling process and requires 

 some starting cause — something to upset equilibrium. It seems that 

 magma transfer on the scale visible in the Appalachians meets the varied 

 requirements. It could start uplift and bring isostasy into play, and it 

 could renew or reverse uplift, as has been repeatedl}^ done. Further- 

 more, the existence of magma bodies with some ease of transfer might 

 permit the relatively small factors of erosion and sedimentation to be- 

 come operative and adjust the crust. Isostasy in the extreme form re- 

 cently advocated demands a mobility of the crust that does not seem 

 possible unless fluid masses are involved in some part of the column. 

 If fluid masses are involved, and unquestionably they have been in the 

 past, it is probable that isostatie compensation is accomplished in them. 

 Its depth would therefore be controlled by the position of the batholiths- 

 and be a variable quantity. 



It is further to be noted that the existence of granite batholiths at 

 depths of 10 or 20 miles does not require an indefinite downward exten- 

 sion of them. Below the zone in which batholiths are known to liave 



