202 F. B. TAYLOR ORIGIN OF THE EARTH's PLAN 



On the same page Suess observes further that — 



"After having described the manner in which the syntactic arcs of the great 

 chains push forward against the north of the Indian Peninsula, we observe 

 that a similar advance of syntactic arcs takes place toward the north of the 

 Pacific Ocean, and that a special tectonic homology exists between that frag- 

 ment of ancient table-land and this part of the ocean." 



From these passages it seems clear that Siiess regarded the Cordilleran 

 ranges of British Columbia as pressing toward the southwest to meet the 

 eastern end of the Aleutian arc pressing toward the southeast, and this 

 is apparently the true relation. There was thus a convergence of crustal 

 movements in the mountain knot of Alaska, and it was the conflict of 

 these movements which intensified the mountain making at the angle 

 and gave the mountain knot its greater breadth and height. Figure 3 

 shows the mountain knot, with part of the Aleutian range which enters 

 it from the southwest and part of the Cordilleran range which enters it 

 from the southeast. The shaded part is 5,000 to 10,000 feet in altitude 

 and the black parts 10,000 feet or more. 



The morainic accumulations of the Pleistocene ice-sheet in North 

 America present homologous forms that are very instructive. These are 

 ihe interlobate moraines which were produced in re-entrant angles of the 

 ice-front, where the fronts of two adjacent great lobes of the ice-sheet 

 came together on converging lines. The mountain knot of Alaska is in 

 a precisely similar sense an interlobate form — a confusion and intensify- 

 ing of mountain making in the angle between two great earth-lobes, or 

 two crustal sheets moving on converging lines. Just as interlobate mo- 

 raines are higher, more bulky, and more tumultuous in form than either 

 of the single morainic ridges approaching the interlobate angle, so the 

 mountain knot of Alaska is broader and higher than either of the ranges 

 approaching it. If the Cordilleran ranges of British Columbia had been 

 folded toward the northeast — that is, away from the ocean — ^there would 

 be no reason for the existence of the mountain knot; and still less if we 

 suppose, further, that the eastern part of the Aleutian range had been 

 folded to the northwest. There would then be diverging movements 

 from the place of the mountain knot — a condition hardly favorable for 

 the formation of such a feature. 



American geologists, following H. D. Eogers, Dana, Leconte, Button, 

 and others in their interpretation of Appalachian structure, made many 

 years ago, derive the thrusts producing the folding of that range from 

 the direction of the Atlantic Ocean. N'o doubt this is correct so far as 

 relates to the immediate thrust forces involved — that is, to those which 



