October 6, 1916] 



SCIENCE 



499 



merit of an outer shell over the internal core, 

 the latter retaining a constant orientation. 

 Astronomical considerations are opposed to an 

 absolute change in the position of the pole, at 

 least during the eras of known geologic his- 

 tory, but they do not interfere with the possi- 

 bility of a relative movement of an outer 

 earth skin, either as a whole or in continental 

 segments, provided there be no effective 

 change in the position of the center of gravity 

 of the spheroid. According to Wegener, whose 

 speculations were published in the Geologi- 

 sches Rundschau and in Petermanns Mitteil- 

 ungen for 1912, the lighter continental masses, 

 floating in denser material of the lithosphere, 

 might move laterally. Postulating the sharp 

 distinction of density and the plastic though 

 resistant character of the substratum, which 

 permits slow movements, there is, says Dacque, 

 no reason to deny that great horizontal dis- 

 placements of the continental masses may 

 occur, if only it can be shown that there are 

 forces which, during prolonged geologic eras, 

 have acted continuously in a constant direc- 

 tion. Finding such a force in deep-seated lat- 

 eral stress due to the effort toward isostatic 

 equilibruim, Dacque concludes that we must 

 hereafter take account of great relative crus- 

 tal displacements with reference to the mass 

 of the spheroid, regarding them, if not as facts, 

 at least as sound working hypotheses. 



It is not the purpose of the reviewer to dis- 

 cuss these concepts, but it may be observed 

 that they may appear reasonable or extrava- 

 gant according to one's previous education. 

 "We have learned to accept horizontal displace- 

 ments of tens of miles. Overthrusts of this 

 magnitude are clearly demonstrated. The gen- 

 erally accepted interpretation of Alpine struc- 

 ture has familiarized European geologists with 

 the thought of much greater horizontal move- 

 ments which are supposed to have resulted in 

 piling slice upon slice of the superficial strata 

 and basement rocks, far in excess of the ability 

 of rocks to transmit crushing strains. Fifteen 

 years ago Lugeon's extraordinary views were 

 regarded as impossible. Now only a very 

 small minority of his colleagues still opposes 

 them, and the general agreement of the mas- 



ters influences the younger generation of Euro- 

 pean geologists, schooled to accept an inter- 

 pretation of mountain structure which contra- 

 dicts the laws of mechanics and physics. 



In a chapter on the rise and sinking of lands 

 or changes of the oceanic level Dacque reviews 

 current theories of the causes of epeirogenic 

 and orogenic movements, as they are repre- 

 sented in the writings of Suess, Wegener, 

 Termier, Laclimann, Andree, Haug, Daly, 

 and others. The tendency is toward an aban- 

 donment of the contraction theory, the assign- 

 ment of a minor role to isostatic adjustment in 

 epeirogenic changes of level, and a return to 

 the old plutonic or thermal hypothesis in some 

 modified form, especially with reference to the 

 subsidence of geosynclines and the subsequent 

 folding and elevation of the accumulated sedi- 

 ments. Alpine studies again furnish the prin- 

 cipal basis of European speculation, but there 

 is also an appeal to English and American 

 thought. 



The permanence of oceanic basins is a theme 

 which Dacque discusses with a full apprecia- 

 tion of its importance in paleogeographic 

 studies and of the diametrically opposite views 

 held by various authorities. After a compre- 

 hensive review of marine transgressions and 

 recessions over continental areas, he cites the 

 arguments for and against permanence of the 

 oceanic basins, and arrives at a sharp con- 

 tradiction of evidence, which he proceeds to 

 solve by adopting Wegener's suggestion of 

 floating continents. It was Suess who desig- 

 nated the lighter rock masses, composed chiefly 

 of silica-alumina rocks, as " Sal " and heavier 

 ones, consisting of silica-magnesia materials, 

 as " Sima." Assuming them to be differenti- 

 ated, sal may be conceived to be a more or less 

 continuous skin floating in sima, and it may 

 be capable of disruption accompanied by sepa- 

 ration of the parts. Sima forms the ocean 

 bottoms and underlies the masses of sal which 

 are the continents. The Pacific is a very an- 

 cient ocean basin; the Atlantic and Indian 

 depressions are young. According to Wegener 

 the Americas have become separated from 

 Europe and Africa, and Dacque finds therein 

 the origin of the intervening deep. He says : 



