I 



THE DRIFTING OF THE CONTINENTS TERMIER 221 



that their roots, flexible and elastic, permit them near the surface 

 to draw nearer to each other; that the oceanic space included be- 

 tween two continents can therefore contract much, for example, from 

 a half to two-thirds of its original width, and then this space, for- 

 merly the sea, becomes a line of folds, a mountain chain which will 

 dominate superbly the continents thus brought nearer together. 

 Geologists differ only in their interpretation of the extent of these 

 phenomena and in the manner of accounting for them. 



Yes; geography is variable. We have known that for a long 

 time. It varies at first a little, very little, I mean to say very 

 slowly, under the action of two powerful leveling agencies — erosion 

 and sedimentation, which tend to equalize the surface of the earth 

 and to establish there the universal ocean, the Panthalasse of Ed- 

 ward Suess — erosion which disaggregates, wears away, dissolves, 

 rounds off, modifies the mountains and very slowly transforms them 

 into a peneplain ; sedimentation which fills the lake basins and tends 

 to diminish the depth of the seas. But geography changes espe- 

 cially by the peculiar movements of the planet which I call its 

 convulsions. These movements are of two kinds, vertical and 

 tangential. The vertical movements displace the shore lines, cause 

 the sea to advance on the continental domain or recede, rejuvenate 

 the monotonous peneplains and worn-down mountains, deeply em- 

 bank the rivers ; elevate or depress the high chains and thus in turn 

 increase and lengthen or diminish the glaciers; raise new moun- 

 tains or plunge beneath the sea portions of continents, vast islands, 

 or entire formations of a formerly majestic mountain chain. They 

 change the conditions of life and of sedimentation at the bot- 

 tom of the sea by changing the depth and the remoteness of the 

 shore. 



These vertical movements are some thousands of meters in extent. 

 They have the singular double character of being oscillatory and at 

 any time counterbalancing each other so nearly that the mean level 

 of the seas does not greatly change. While certain points of the 

 surface rise some thousands of meters and others sink simultaneously 

 a comparable amount, the mean level of the sea in comparison 

 with the supposedly fixed mean level of the continents varies only 

 a few hundred meters. 



The tangential movements are those which wrinlde the surface of 

 the earth, transforming an entire section of the surface into a sys- 

 tem of folds, which needs only elevation to become a chain of moun- 

 tains. The extent of the tangential movements is indeed far greater 

 than that of the vertical ; for a particular chain it is some hundreds 

 of kilometers, if indeed it does not attain to a thousand. The chain 

 of the Alps, for example, whose width, to-day, scarcely exceeds 

 250 kilometers, resulting from the contraction of an elongated zone 

 20397—25 16 



