REPORT OF THE DIRECTOR 1920-21 69/ 



by continental fragmentation, by which process he conceived the 

 margins of the continents to have been faulted down into periodi- 

 cally widening oceanic basins. This hypothesis has found strong 

 adherents in this country, as Hobbs (1907, p. 233), who, by his 

 seismological investigations, believes to have found evidence that 

 " the ocean- basins of the present day have been formed largely 

 as a result of sinking of great orographic blocks." He thus disputes 

 the permanency of the present ocean basins. On the other hand, 

 Schuchert (1916, p. 91) inclines to the view that the enlarging oceanic 

 basins are the most permanent features of the earth, while the 

 ancient continental platforms originally were not only arranged 

 differently (latitudinally) than at present (longitudinally), but 

 also that their areal extent, including their emerged and submerged 

 portions, was originally greater than at present; and vast land 

 masses, such as Oceania (Australia, New Zealand) and Gondwana- 

 land have permanently been taken possession of by the oceans. 



Suess's hypothesis has been subjected to critical analysis by 

 Barrell (Nature and Bearings of Isostasy, 19 19), who points out 

 that Suess formed his theory of continental fragmentation before the 

 theory of isostasy had been developed and proved by the quantita- 

 tive data now available. " These data indicate that a continental 

 platform can not break down broadly into an ocean unless there has 

 been a previous or accompanying increase in density in the litho- 

 sphere. Such increase in density might be made locally by the rise 

 of great masses of basic or ultrabasic magmas, but there is no 

 independent geologic evidence that this has occurred on the scale 

 demanded by the theory of Suess, nor that it could be effective on 

 such a scale." Barrell does, however, not exclude the possibility of 

 broad density changes and concludes that if the geologic evidence 

 of continental fragmentation should become regarded as compulsory, 

 the conditions of fragmentation and isostatic compensation might 

 conceivably be reconciled by the existence of a third and otherwise 

 unsuspected condition of changes of density in tne lithosphere. 



Chamberlin, in his classic work The Origin of the Earth and in 

 other publications, has advanced an independent theory according 

 to which the origin of continents and oceans dates back even to the 

 middle growth-stages of the earth and the latter is believed to have 

 begun to hold an ocean by the time it contained 30 or 40 per cent 

 of its present mass. Through various influences, notably those of 

 wind and water, the denser planetesimal dust tended to be segregated 

 into the primitive ocean basins, while the lighter dust accumulated 



