878 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



ward movements were started, tlie domed shape of the sea-floor would 

 become a factor and would transmit lateral thrust in all directions from 

 the center of depression. The distance to which this thrust would be 

 transmitted would be in proportion to the thickness and strength of the 

 crust which was depressed. If the depressed portion were 40 or 50 miles 

 thick, a horizontal transmission of even a thousand miles would be rea- 

 sonable. A phenomenon of this order is comparable with those of the 

 Appalachians, and a depression near the middle of the N"orth Atlantic 

 would affect regions at the distance of the Appalachian Mountains. It 

 is from possibilities of this sort that the hypothesis of suboceanic thrust 

 finds its support, as well as in the general arrangement of mountain 

 chains near the ocean margins. 



To carry the analysis still farther, let it be assumed that the ocean- 

 floor settled and thrust against the margins of the continents. This 

 thrust would not be transmitted indefinitely, but would somewhere reach 

 a weak belt. Yielding to the strain would ensue, and either folds or 

 faults would be caused. 



Let it be further borne in mind that there are reservoirs of molten 

 matter at some depth in the crust. The positions of these were doubtless 

 adjusted to the general set of pressures prevailing before the first sub- 

 sidence began. Xew and greater pressure on them would certainly cause 

 changes of position, fresh channels would be forced open by molten ma- 

 terial, and these must needs tend to spread toward the regions where 

 pressure had been released around the faulted sea margins. Once the 

 system of adjusted j)ressures was upset, the mechanism of batholithic 

 intrusion was put into action, with the 2orobable result of deformation 



A general subsidence of the western Atlantic floor l^y one mile would 

 expel 4,000,000 cubic miles of material. This amoimt, translated into 

 batholiths and folding, is enough to account for the lateral shortening 

 of a mass 2,000 miles long, 10 miles deep, and 400 miles wide into half 

 its width. In short, so far as quantity is concerned, a subsidence of the 

 Atlantic floor by one mile could produce the Appalachian system. Such 

 a subsidence is^ moderate one, and movements of such magnitude have 

 not been rare in the geologic past. On the contrary, oscillations of the 

 sea-floor have been so numerous that they may be regarded as a normal 

 characteristic, from the beginning of the Paleozoic to the present. 



The force available to propel the intrusive masses and effect the fold- 

 ing was that of gravity, and its measure was the difference in density 

 between the oceanic and continental jjarts of the earth's crust, perhaps 

 as much as 5 per cent. When this is translated into terms of weight for 

 columns of ten or more miles in depth, it is truly a formidal)le force. 



