THE DRIFTING OF THE CONTINENTS TERMIER 227 



everywhere with the same exuberance and variety of vegetation 

 which covers them. It is admirable ; but in order not to be misled it 

 is necessary to remember what I said a moment ago of the danger of 

 these very convenient hypotheses which make appeal to occult forces, 

 perhaps inexistent, and which afford us the formidable power of dis- 

 posing at will of the continents and seas. 



Such is Wegener's theory of the drifting of the continents, with 

 its undeniable attractions and deceptive power, probably mislead- 

 ing. It was received with very great enthusiasm, at first, especially 

 among geophysicists and in Germany. The inconsistency came 

 later. It is a curious thing that, in spite of the latitude which this 

 theory gives to geologists, there are those amongst them who have 

 been unwilling to accept it and are the ones who have raised the 

 strongest objections. Let us consider the matter a little more closely. 



Wliat could be this fluid magma on which the continents float and 

 in which their bases are immersed? According to Wegener, it is a 

 bath of melted rock having almost the composition of the heaviest 

 lavas emitted by our volcanoes. You know that lavas vary from one 

 volcano to another and sometimes even in the same volcano from one 

 eruption to another. There is a whole gamut of lavas, the lightest 

 of which, richest in silica and oxygen, when solidified are of the 

 lightest color and have very nearly the chemical composition of 

 granite, while the heaviest, the most basic, charged with magnesia 

 and iron, have a much darker color and are even at times entirely 

 black; these are called basalts. From one extremity to the other 

 of this gamut the density of the lava, supposedly solidified and com- 

 pletely crystallized, varies in round numbers from 2.8 to 3. It is 

 a very remarkable fact that the gamut of lavas has not changed 

 since the most distant geological epochs of which we have any 

 knowledge. Even in pre-Cambrian times, probably already when 

 life began, there were volcanoes which ejected lavas in every way 

 resembling those of the Tertiary volcanoes, some that are called 

 rhyolites, having the chemical composition of granite, others that 

 were basalts, identical with our basalts, and a series of others of 

 intermediate composition. Hence the idea that there is all around 

 our globe, under the solid crust, or lithosphere, a liquid spherical 

 zone, or pyrosphere, from which the volcanoes have always been 

 and still are supplied; that, since the beginning of geological time, 

 the mean limit of the lithosphere and the pyrosphere have not sensi- 

 bly varied; that, periodically, but with a periodicity unknown, on 

 a vertical scale this limit rises or descends, but without exceeding a 

 maximum value, neither does it descend below the minimum value 

 of its distance to the center of the globe. On the other hand, all 

 the geophysical data make us believe that, in the interior of the 

 earth the elements are classed, at least approximately, in the order 



