490 BOUNDARIES OF THE SEA 



ocean, but the percentage of quartz sand in the geosyncUnal 

 troughs is uncomfortably high and that of volcanic ash unen- 

 couragingly low for this explanation. Neither does the small depth 

 of deposition one must assume for most of the sediments recall 

 the usual deep basins between the present island arcs and the 

 Asiatic continent. There are some cases, like that of New Zealand, 

 in which this suggestion of an island arc is no help, because it 

 would have to be placed right in the deep ocean. Hence, I still 

 consider the case a strong one for borderlands that have disap- 

 peared. 



Perhaps it is time to explore a possibility that is contrary to the 

 usual way of thinking. One must admit that there are se^•eral inde- 

 pendent arguments pointing in the same direction that the deep- 

 sea floor is a bottomless tub into which continental matter is ever 

 disappearing. I see no compelling arguments for the popular 

 notion that the continents have grown in the course of geological 

 history, because to my knowledge old land surfaces are invariably 

 found where the base of sedimentary prisms are exposed. All I am 

 prepared to admit is possible growth of consolidated shields inside 

 the borders of the continents. But a mountain range born from 

 sediments deposited along but outside the continental blocks on 

 the deep-sea floor still has to be found. 



To substantiate the claim of insatiate oceans we have to con- 

 sider, on the one hand, the inferred disappearance of former con- 

 tinental terraces and source areas of sedimentation, and, on the 

 other hand, the absence of deep-ocean sediments on the continents 

 (apart from Timor). Then there is geological evidence — to my 

 mind unquestionable — that basins like the Mediterranean and the 

 trenches of the Moluccas were produced by subsidence in the late 

 Tertiary. Neither can one doubt the gradual subsidence of atoll 

 foundations and uncrowned guyots. The submarine canyons off 

 California with their partly rocky walls and the borderland 

 topography of basins and ridges in that area strongly favour 

 major subsidence of crustal blocks on the continental margin. 

 Estimates of the thickness of oceanic sediment covering the deep- 

 sea floor may not be very trustworthy. But the thickness measured 

 seismically is so far below what one might expect, that it is tempt- 



