DERIVATION OF THE FLORA AND FAUNA 327 



mation of the geanticline structures into rows of islands is explained by Du ToiT 

 as follows; their crest 



could become deepened by crustal tension and broken into segments to form an island 

 chain before vanishing . . . limbs were intermittently built up and destroyed during the 

 Cretaceous-Tertiary through stretching in the direction of their length while they were 

 still compressed by forces at right angles thereto (p. 293). 



Trying to apply these ideas to South America, which according to Du ToiT 

 as well as WEGENER was pressing into the Pacific basin, the Juan Fernandez- 

 Desventuradas-Merriam ridge could be compared to an advance-fold. But it is not 

 convex to the Pacific, nor fronted by a fossa — this is situated on the wrong 

 side and may well stand in causal relation to the upheaval of the Cordillera. To 

 think that the submarine ridge emerging in the Juan Fernandez and Desventu- 

 radas Islands is the easternmost advance-wave from a western borderland seems 

 too phantastic. 



Du Toit's idea of the geological character of the ocean floor differs from 

 Wegener's. Seismographic records, he says, scarcely bear out that the Pacific 

 floor must be composed of basalt — the records could readily accord with a 

 granitic layer up to about 10 km thick (p. 212). He was no believer in a more 

 or less unlimited oversea migration of plants and animals, nor in land-bridges, 

 and he critizises J. W. GREGORY and the bridge-constructing biogeographers: they 

 are wrong, and the displacement hypothesis interprets otherwise. But when he 

 speaks of the extensive "march into the ocean of crustal waves, thereby leaving 

 their parent continents far in the rear" and of the "rhythmic intrusion, culminat- 

 ing in the three migrations of the Cretaceo-Eocene, mid-Tertiary and late Ter- 

 tiary" (p. 214), these advance-folds, when crumpled up from the ocean floor, were 

 absolutely devoid of every sign of terrestrial life and without a trace left of the 

 sial cover. I fail to see that they can solve any biogeographical problems — we 

 have to fall back on oversea dispersal. Wegener's festoons were at least split 

 off from the borders of a continent and left behind with their fauna and flora. With 

 regard to Juan Fernandez we shall perhaps be able to find a less adventurous 

 explanation of its history. 



Two years after the appearance of his book, Du ToiT summarized his theo- 

 ries in a paper which I think it is worth while to quote [82. 75-76). The base- 

 ment of the Melanesian islands is, he says, for the most part continental; the 

 ocean floor consists of a relatively thin structure of sial underlain by sima, but 

 this does not allow us to regard the sial as continental, because it may be a 

 product of magmatic differentiation from the sima. He points to the parallelism 

 between the great Tertiary folding-zones, most evident along the west coast of 

 the Americas, and the trend of the coast line, and he thinks that the "compres- 

 sive phases" were contemporaneous all around from New Zealand across Antarc- 

 tica to South America. Coming back on the advance-folds he remarks that some 

 of Gregory's hypothetical bridges or land-masses could well have been of this 

 nature. The procedure is illustrated by a map showing the pressure direction 

 and the formation of island arcs — except on the American side, where the sea 

 is a blank. 



