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SCIENCE 



[N. S. Vol. XXXI. No. 790 



deepened enough to affect the extent of 

 epicontinental seas. And no ocean basin 

 has been compressed, crumpled and raised, 

 after the manner of the Appalachians or 

 Alps. 



The structural relations of geosynclines 

 are intra-eontinental, those of oceans are 

 extra-continental. The geosyncline occu- 

 pies a position among the positive conti- 

 nental elements. The oceanic basins sepa- 

 rate and snrround continents. 



The distinction between geosynclines 

 and ocean basins is thus fundamental, and 

 to reason from the history of the one to 

 that of the other, as has sometimes been 

 done, is necessarily misleading. 



This distinction noted, we may return to 

 the proposition that the ocean basins have 

 always been permanent since ocean waters 

 gathered. 



The evidence is clear and unquestioned 

 that marine waters have circulated and 

 marine faunas have migrated from epi- 

 continental seas of the eastern or western 

 hemisphere to those of the other hemi- 

 sphere, and they could only have done so 

 across or around bodies of water occupying 

 the sites of the present oceans. 



We have good reason to assume that the 

 volume of oceanic waters has not changed 

 materially from what it was at the incep- 

 tion of existing conditions, it being appar- 

 ently true that contributions from within 

 the earth have been relatively small dur- 

 ing geographic eras, and none being known 

 from without. 



The ocean basins are now somewhat 

 overfull ; they are not large enough to hold 

 all the waters, which therefore extend 

 over the margins of the continents. Dur- 

 ing certain epochs of the past the waters 

 have spread farther, the basins having 

 then been less capacious ; again during cer- 

 tain other epochs the waters have with- 

 drawn into deeper or wider basins. These 



variations have lain within narrow limits 

 as compared with the total volume of the 

 oceans, and they have occurred repeatedly, 

 in alternation. Had a continent ever ex- 

 isted in place of one of the ocean basins, 

 it must on sinking to oceanic depths have 

 produced a disturbance of these nicely ad- 

 justed relations, of which the geologic 

 record shows no trace; which must, how- 

 ever, have been of such magnitude that it 

 would have marked off an older era of 

 small lands from a later one of great con- 

 tinents. No such event has taken place, 

 and no continent of oceanic extent has 

 sunk to oceanic depths. 



This conclusion bears on the reconstruc- 

 tion of former continental extensions. If 

 we accept the evidence that Appalachia 

 formerly extended southeastward into the 

 Atlantic, we must consider reasonable 

 limits. If we erect a transatlantic land to 

 connect Africa and South America, or 

 postulate a Gondwana land from Africa 

 to Australia, we must provide for the 

 waters which such lands displace. The 

 ocean basins and possible epicontinental 

 seas are the only refuge for the waters 

 which are thus hypothetically evicted, and 

 their capacity may be overtaxed. 



The capacity of a basin being affected 

 by changes in depth or width, it is obvi- 

 ously possible to argue that narrower but 

 deeper basins may formerly have contained 

 the waters that are now held in wider and 

 possibly shallower ones. To a certain ex- 

 tent this view may be entertained, but it 

 has limits and they are close to present 

 conditions. The average depth of nearly 

 two thirds of the ocean's basins below the 

 continental plateaus is 4,000 meters or 

 more. At this difference of altitude the 

 weight of the continental column crushes 

 its base and creep ensues. The depth can 

 not be materially increased without occa- 

 sioning corresponding spreading and low- 



