DISCOVERY 



115 



shown by the large and growing literature on the 

 question. 



It is impossible in so little space to discuss the 

 evidence which confirms the theory ; this evidence 

 will be found in my book Die Entsiehung der Kontinente 

 und Ozeane.^ It must suffice here to give a few of the 

 main lines of proof, drawing examples from each of the 

 sciences concerned. 



In drawing up statistics of the distribution of levels 

 over the land surface and sea bottom, geophysicists 

 have found that these heights are grouped about two 

 well-defined values, a land height of about loo metres 

 and a sea depth of about 4,700 metres. (Compare 

 Fig. 2.) This law has been known for fifty 3'ears, so 

 far without any explanation. If the heights and 

 depths had arisen through elevation and depression of 

 a single initial level, as geology has hitherto assumed, 

 then we should expect statistics of level to show a 

 grouping about a single mean value, as shown by the 

 dotted curve in Fig. 2. Instead of this there is a group- 

 ing about two values. So we must suppose that there 

 are two initial levels, on which the elevations and 

 depressions have been superimposed ; and this is only 

 possible if these initial levels correspond to two different 

 layers of the bod}' of the earth. The continental masses 

 consist of comparativelj- light material (such as granite 

 and gneiss) extending downward, according to Hayford 

 and Helmert, to a depth of 100 kilometres. The deep 

 sea bottom is apparently composed of heavier material 

 (such as basalt), in which the continents float like 

 great ice-floes in water. The results of measurements 



' Not yet translated into English. Published by Vieweg & 

 Son, Braunschweig. 2nd Edition, 1920. 



of gravity, and of magnetic and seismic investigations, 

 are in agreement with this conception, and the results 

 of dredging do not contradict it. 



Geology provides a very searching test of our sup- 

 position that the Atlantic is really an enormously 

 widened rift. If this is the case, the mountain folds 

 and other geological structures which existed before 

 the separation must correspond when we bring the 

 continents together again and reconstruct their original 

 relative position, just as the lines of a torn drawing 

 would correspond if the pieces were placed in 

 ju.xtaposition. This is actually the case ; the Permian 

 folds of the Cape mountains fit exactlj^ to the Sierras of 

 Buenos Aires, which, according to the latest work of 

 the Argentine geologists, are of the same age and have 

 a completely similar structure. The distance of these 

 mountains from the Cameroons on the one side, and from 

 Cape San Roque on the other, is the same, so that they 

 fit each other exactly in the reconstruction. The direc- 

 tion of folding in the great gneiss plateau of Brazil also 

 corresponds with that in the opposite regions of Africa. 

 In Europe there are three ancient mountain chains 

 which arose in the Silurian, Devonian, and Carbonifer- 

 ous epochs, and these mountain chains are so placed in 

 North America that they appear in the reconstruction 

 as undoubted continuations of the European system. 

 The terminal moraines of the Great Ice Age also appear 

 now as a continuous system. The most striking fact 

 is not the existence of the same features across the 

 Atlantic, but their situation at places which correspond 

 exactly. For example, if the Sierras of Buenos Aires, 

 which are now more than 6,000 kilometres distant 

 from the Cape mountains, lay only a few hundred 



2 The geological terms in general use to indicate long periods of past time are tabulated below. As matters of interest, 

 examples of the rocks formed during the period and prominent life-forms of the period are included in the table. In the fifth 

 column are given estimates, necessarily rough and speculative, taken from Dr. Wegener's book, of the number of years between 

 the beginning of the period and the present day. 



ERAS AND PERIODS OF GEOLOGICAL TI.ME 



