328 



C. SKOTTSBERG 



Numerous biologists have found a solution of all or most of their difficulties 

 to explain the present distribution of the organic world in the theory of conti- 

 nental drift, combined with large-scale pole-wanderings. In view of this the opi- 

 nions expressed by modern geologists and physiographers cannot be passed in 

 silence. A symposium, arranged in 1950 (dj) offers an opportunity to get ac- 

 quainted with their attitude. 



J. H. T. U.MI!GR0\E, The case for the crust-substratum theory, pp. 67-71. 



Sless' terms sial and sima were petrographic. WEGENER attributed different 

 physical properties to these types of rock; sial should be rigid but elastic, sima 

 viscous. These statements lack foundation. Sima (basalt) has a higher melting- 

 point, approximately I300°C, sial (granites) approximately 700°C. The crystal- 

 line crust of the sima layer is at least as strong as the continental sial. Still the 

 sial blocks were supposed to advance through the sima. U.MBGROVE concludes 

 that continental drift is impossible at present. This granted, was it perhaps pos- 

 sible in bygone times.' The answer is fetched from the Atlantic and Indian oceans 

 with the intervening African continent; if the floor of the oceans originated as 

 thought Wegener, the processes must have taken place during early Precambrian. 

 Umbgrove asks if not the thick blanket of sediments would have been squeezed 

 and piled up in front when America ploughed westward. He calculates that, 

 considering the size of the westward drift, a plateau 200 km wide at sea level 

 would have been formed in front; instead, "the continental slope is one of the 

 steepest in the world and is fronted by deep-sea troughs" — here we have, how- 

 ever, to consider the late upheaval of the Andes. And if, as Wegener's theory 

 requires, the Atlantic originated in comparatively recent times, how are we to 

 explain the enormous thickness of its bottom sediments, according to Hans Pet- 

 TERS.SON a maximum of loooo feet, "representing a time-span of 300 to 400 

 million years", which would bring us back to the Palaeozoic. If continental drift 

 ever occurred, Umbgrove asserts, it took place some 3000 million )'ears ago and 

 consequently loses every shade of interest to the biogeographer. 



Harold JefereVS, JMecJianical aspects of co)itinental drift and alternative theories. 



Jeffreys definitely rejects Wegener's theory on geophysical grounds; 

 Wegener gave to the sima properties which the material has not; basic rocks 

 are stronger than acidic, sima stronger than sial. The strength of the ocean floor 

 must be overcome, if drift shall result. "I seriously suggest", he concludes, "that 

 no more time be spent on discussion of this theory until a mechanism for it is 

 produced; what it has done, and continues to do, is to distract attention from 

 the serious problems of geophysics" (p. 80). 



S. W. Wooldridge, The beari7tg of Late-tertiary history on vertical and hori- 

 zontal movements of the continents. 



The nature of the mechanism of the "uplift" may remain in doubt; its reality can 

 hardly be gainsaid ... in so far as such complementary foundering took place during 

 Tertiary times, it is not directly relevant with the drift problem . . . Vertical movements 

 offer an alternative solution to some of the biological problems. Biological evidences 



