2o6 WILLIAM H. HOBBS 



The ocean floors are today for the most part sinking and 

 represent the great areas of dispersal of thrusts toward the neigh- 

 boring continents, and mountain arcs should therefore be convex 

 toward oceanic depressions. If in the past during mountain- 

 making periods ocean floors have represented areas of dispersal of 

 thrusts, the position and the orientation of the earlier arcs should 

 be an expression of the former areal relations of the continental 

 pedestals and the ocean floors. For the continent of Asia this 

 theory appears to be borne out by what we know of continental 

 evolution within the principal mountain-making periods. 



The active force (thrust) which produces rock folds, instead of 

 operating from behind and above the anticline, as so generally 

 supposed, is apphed below and in front. Continuation of the 

 process yields therefore not "overturned" and "over thrust," but 

 underturned and under thrust flexures. Applied to the Alps, this 

 requires that the main active force concerned in their folding came 

 from the northwest, instead of the southeast, as generally assumed. 



Anticlines arise first upon that side of the folding area which is 

 toward the direction from which the force comes. In strong or 

 competent members anticHnes lift a portion of the load from arched 

 underlying formations, and this portion, regarded as a percentage of 

 the total load upon the arch, is at first small, but afterward rises 

 steadily and rapidly up to the stage of underturning, after which it 

 rapidly diminishes. 



With the underturning of an anticline a new distribution of 

 stresses is inaugurated, as a result of which a second anticKne may 

 develop behind the first and subsequently others in succession but 

 of steadily diminishing dimensions until the series comes to an end. 

 Thus it comes about that while the arcs in the order of their age 

 develop from within outward in the series as the continental area 

 becomes extended peripherally, the folds (anticlines) within any arc 

 are developed in order of age from without inward (Fig. 39). 



With the stage of underturning a new couple composed of vertical 

 forces enters and tends to rotate the underturned arch, not forward 

 as before, but downward; and if the competent member is over- 

 laid by a weak formation, the crown of the arch will be left without 

 sufficient support and will sink to form a "plunging crown." If 



