and Diastrophism in Oceanica. 93 



rising strand-lines resulting from the melting of the ice. On 

 the other hand, the Pleistocene stranddines remained neutral 

 at about 35° north and south latitudes; further poleward they 

 were positive, due to the gravitative power of the great masses 

 of polar ice. 



Permanency of continents and oceans. — It is now more than 

 fifty years since James D. Dana began to teach that the rising 

 continents and the sinking oceanic basins have been, in the 

 main, permanent features of the earth's surface. He did not 

 mean, however, that the continents have always had essentially 

 the same shape, elevation, and areal extent that they have 

 to-day. Still, Dana did not fully appreciate the amount of 

 continental fragmenting that has taken place in the course of 

 geologic time, though he clearly pointed out the foundering 

 of Australasia, speaking of it in his famous Manual of Geology 

 (page 797) as "a fragment of the Triassic world." The 

 teachings of Dana as to the permanency of continents and 

 oceanic basins have been accepted in some form by all geologists, 

 and lie at the basis of all zoogeography and evolution as well. 

 In Dana's time and to some extent even to-day geologists are 

 swayed by the Wernerian or Neptunian theory of earth history, 

 which postulates a gradual emergence of the land out of the 

 decreasing hydrosphere through loss of water by crustal absorp- 

 tion. Wow, however, geologists are holding more and more to 

 the hypothesis that the earth periodically shrinks, and each time 

 it does so some parts or all of the continents rise more or less; 

 but that in the main there is subsidence of the ocean bottoms 

 equal in amount to the rising land-masses, that the water of 

 the hydrosphere is constantly increasing in amount, and that 

 even though the continents are in the main permanent, yet 

 they are partially breaking down into the oceanic basins. 



From this we conclude that the enlarging oceanic basins are 

 the most permanent features of the earth's surface. On the 

 other hand, along with the progressive subsidence, the bottom 

 of the Pacific is also built up into many local volcanic cones 

 by outpourings of lava, and further, it rises into more or less 

 long mountain ridges. Some of these elevations of the bottom 

 appear at the surface of the ocean as groups or lines of dead 

 or active volcanoes (see fig. 1). Another general conclusion is 

 that most of the "deeps" of the Pacific Ocean situated between 

 18,000 and 31,800 feet beneath the surface occur near the con- 

 tinents that exist now or existed formerly, or that they are 

 located on the outer or oceanic side of mountain chains. These, 



