84 WILLIAM H. HOBBS 



processes have not yet succeeded in reducing their excessive relief. 

 Still farther outward and upon the margins of the system, we have 

 mountains which may more properly be described as in the making, 

 with all the accompaniments of strong earthquake and violent 

 volcanic eruption. 



The ocean basins the loci of dispersion of tangential thrusts. — To 

 this conclusion we are led by the considerations of the last section.^ 

 To the tectonic argument modern seismology has made a con- 

 tribution of the first importance. Until within a quarter of a 

 century it had been the custom to regard the continents as the 

 regions of the most active geological movement upon our planet — 

 a striking illustration of the assumed dominance of phenomena 

 near at hand and often observed in fixing the basis of judgment. 

 Yet a moment's thought shows us that the floors of the oceans 

 are masked beneath a mobile cover which must eft'ectively damp 

 all disturbances that emanate from them. Now that a means 

 has been discovered for seismically exploring this vast area and 

 subjecting its movements to measurement, we have been surprised 

 to learn that its mass movements are, area for area, vastly greater 

 than those of that portion of the lithosphere which projects above 

 the seas. 



If we except the immediate vicinity of the continents, there 

 is evidence that as a whole the movements upon the ocean floor 

 are downward toward the earth's center, and hence the greatest 

 reduction in superficies is there today in progress. Of this the 

 evidence is twofold. By far the greatest of all macroseisms 

 proceed from the deeper portions of the ocean floor, and we may 

 draw the conclusion that if the mass movements were not in general 

 downward rather than upward, these areas could hardly remain the 

 deeps. On the other hand, except in the neighborhood of great 

 deltas, most mountain shores of the continents are today rising. 

 This is eminently true of that remarkable fringe of island arcs 

 which lie to the eastward of the Asiatic continent, a fact amply 

 demonstrated by the elevated shore lines and high coral reefs. 

 The amounts of elevation are here further in direct proportion to 



' See also T. C. Chamberlin and R. D. Salisbury, Geology, I, 517-18, 520-21. 



