FRONTAL OCEANIC DEEPS 201 



which stand adjacent to them. They are of the nature of sunken or 

 depressed forelands, and are apparently due to the stupendous weight 

 and pressure of the adjacent ranges. These arcs are no doubt more or 

 less overthrust upon the ocean floor, and the troughs are probably due in 

 part to elastic yielding and perhaps in part also to plastic flow. 



The most strongly marked deeps are close in front of island arcs which 

 represent submerged mountain ranges. Evidently the reason that the 

 Tuscarora and Aleutian deeps remain unfilled today is that the ranges 

 to which they are related have remained submerged, have suffered little 

 or no erosion, and hence have supplied very little sediment. Other simi- 

 lar frontal depressions situated close to continental lands which supplied 

 great quantities of sediment have been partly or wholly filled. Such are 

 the valleys of the Ganges and Indus in India, the Tigris-Euphrates Val- 

 ley and the Persian Gulf, and also the Adriatic Sea and the Po Valley. 

 Such troughs, growing deeper while sediments are being deposited in 

 them, furnish a possible explanation of certain sedimentary strata whose 

 great thickness and shallow-water character seem to demand subsidence 

 during deposition. 



The fact that the greatest deeps, both unfilled and filled, lie close to the 

 front of the peripheral ranges of Eurasia, the greatest of the continental 

 units, adds one more significant group of facts to the great aggregate, 

 showing how much more vigorous were the Tertiary crustal movements 

 there than in any other part of the world; and they join with the other 

 evidences mentioned above, which show a general southward crustal 

 movement for the whole of Eurasia. 



The Eelation of North America to Eurasia 

 the mountain knot of alaska 



In his earlier writings, Suess regards North America as a continental 

 unit which moved in harmony with the Tertiary movement of eastern 

 Asia — that is, it was folded toward the Pacific Ocean. Eeferring to 

 North America, Suess says: ^'So far as folding is known in this continent, 

 it ap]:)ears to be everywhere directed to the west^^ (I, 600). 



It may be observed here, however, that the Tertiary folds bordering the 

 Pacific are mainly folded toward the soidJucest, rather than to the west, 

 as Suess states, and that only in the States of Oregon and Washington 

 are thev folded to the west. 



