December 16, 1922J 



NA TURE 



799 



the past. Like many scientific workers, he feels that 

 a recognition of the Permo-Carboniferous ice-age 

 compels him to put forward an explanation. Like 

 them, he overlooks the fact that a century of specula- 

 tion as to the causes of the glacial epoch of far more 

 recent times has left us with a score of hypotheses 

 amid which we wander unconvinced. The evidence 

 of the occurrence of ice-ages becomes more and more 

 cogent as observation spreads, and it is highly probable 

 that they have a common cause. Prof. Wegener, 

 in laying stress on the differences between equatorial 

 and polar temperatures at the present day, takes up 

 the position of greatest difficulty, and regards a re- 

 gional refrigeration as necessarily connected with the 

 poles. He does not look beyond our planet and the 

 atmospheric conditions that now prevail. It is evident 

 that Prof. Spitaler's laborious inquiries as to zonal 

 fluctuations will not content him, though this author 

 believes that he has drawn the Permo-Carboniferous 

 glaciation into his uniformitarian net. Wegener's 

 suggestions are far more heroic ; he will shatter the 

 outermost layer of the crust to bits, and remould it, 

 by successive arrangements of the pieces, nearer to 

 his heart's desire. His theme is fascinating, and his 

 style is admirably lucid. His fondness for " huben 

 und driiben," a phrase, we believe, derived from 

 Goethe, makes us wonder if he treats the globe as 

 lightly as it was treated in the " Hexenkiiche." For 

 him indeed " sie klingt wie Glas ; sie ist von Ton, es 

 giebt Scherben." 



As is well known, Wegener has been much impressed 

 by the easterly salient of S. America and the easterly 

 indent of the African coast. If we could assure 

 ourselves that these were at one time, and at the 

 right time, actually in contact, most of the problems 

 of oceanic islands, of palseoclimatology, and of the 

 distribution of land-organisms, would be solved. 

 Would not the instability of S. America in regard 

 to Africa imply a similar instability between N. 

 America and Europe, of which there is (p. 81) some 

 geodetical evidence, accepted by Wegener, but much 

 open to discussion ? If the Atlantic is a crustal rift, 

 the other oceans are likely to have had a similar 

 origin. The primary crust, the silica-alumina layer, 

 which Wegener calls sial in preference to Suess's less 

 distinctive word sal, broke open and gave rise to con- 

 tinental blocks and accessory islands, which float, and 

 even waltz, upon the sima, the silica-magnesia layer 

 that underlies them. 



There is a concluding figure in many Bantu dances — 

 it survives even in folk-dances at Skansen — where two 

 partners turn back to back, bump, and part again. 

 The possibility of this figure on' a continental scale is 

 thrilling and attractive. If Africa once parted from 

 NO. 2772, VOL. I io] 



America, she may woo her mate again as years pass 

 by. The hand of the philosopher may be laid on 

 the great land-blocks, and the occurrence of Glos- 

 sopteris in India or of Geomalacus maculosus in 

 Kerry may be explained by a simple process of 

 " Verschiebung." If the fitting is not sufficiently 

 accurate, some plasticity is granted to the sial blocks, 

 and " Umwalzung " is also possible (pp. 35 and 41). 



Wegener's conception, however, must not be taken 

 in the spirit of a jest. Experiments on the force of 

 gravity, made over very wide areas, have established 

 the existence of a mass-defect under mountain-ranges 

 and a mass-excess under lower grounds and oceans, 

 and the sea-floor may be justly regarded as consisting 

 of sima in large degree. It has long been recognised 

 that a crumpled crustal mass bulges both upward and 

 downward ; it displaces what we now call sima in the 

 depths. On the theory of isostasy, it maintains its 

 elevation above the general surface by the fact that 

 it displaces matter the specific gravity of which is 

 greater than its own. Like ice in water, it floats, with 

 a certain portion unsubmerged. 



The analogy with ice is seized on by Prof. Wegener. 

 If icebergs shift their places and " calve " by cracking 

 on their flanks, why should not continents do the same ? 

 Let us grant that the level of the sima is reached at a 

 less depth than that of the ocean-floors ; the latter 

 must then be composed of sima, and over them the 

 buoyant continents may meet, and waltz, and part 

 again. Of course they may do so ; but when we are 

 asked (p. 101) to look for the sima level about 100 

 fathoms down, or in some rare and dubious cases at 

 250 fathoms, we find that the rocks familiar to us on 

 the land-surface are held to extend very little beyond 

 the ordinary continental shelves. The chalk and flints 

 dredged from 600 fathoms off western Ireland will 

 require a new explanation. In depth, the continental 

 blocks may go down to 100 km. Their relations to 

 the earth as a whole, on this supposition, are shown on 

 the same longitudinal and vertical scale in an expressive 

 section following a great circle between S. America 

 and Africa. The two continents are seen to be well 

 immersed in sima. Sima (p. 113) behaves under 

 pressure like sealing-wax, and sial like wax. Hence 

 crumpling occurs in the sial blocks when they are 

 pressed against the sima, though the latter in time 

 yields and flows. Higher temperature in the depths 

 assists this flow, and (p. 105) inclusions of sima in the 

 base of sial blocks assist, by their greater fluidity, the 

 yielding of the sial under folding thrusts. 



We have now before us Wegener's view of the possi- 

 bility of great horizontal displacements of the con- 

 tinents. The author points out (p. 6) that H. Wett- 

 stein in 1880 regarded the continents as subject to a 



