222 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



for a long while occupied by the sea, was formerly at least 800 

 kilometers wide, perhaps a thousand. The southern margin of this 

 zone has approached the northern margin, and the sediments ac- 

 cumulated on the bottom of this elongated sea have been folded 

 and even refolded upon each other. If, instead of the Alps, we con- 

 sider the entire system of the great chains of central Asia which is 

 actually 3,000 kilometers from north to south, we are led to con- 

 clude that the transverse sea roughly parallel to the Equator, was 

 formerly at the very least 6,000 kilometers in width for, in this sea 

 which the geologists call Tethys were deposited the sediments which 

 constitute the major part of these chains. It was a sort of trans- 

 verse Atlantic; it has gradually narrowed by the contraction of its 

 margins, a contraction which has proceeded without doubt by suc- 

 cessive abrupt movements, whose total extent embraced eight or nine 

 of our geological periods, that is to say several hundreds of millions 

 of years. Such is the common teaching of geologists on the defor- 

 mation of the surface of the earth ; and you see that they all play a 

 large part in the variations of geography and in the mobility of the 

 lithosphere. 



But very recently, in 1912, a German geophysicist, Alfred 

 Wegener, conceived the idea of a very much greater mobility. He 

 uprooted the continents and compared them to pontoons floating 

 to a port, of which I was just speaking, or, better still, to those ice- 

 bergs which each spring are born by the breaking up of the polar 

 ice flow and, carried away by the polar currents, pass toward the 

 temperate regions of the ocean, a white flotilla dreaded by navi- 

 gators. These mountains of ice drift with a speed varying slightly 

 according to their form and dimensions, and the inequality of their 

 progress soon exaggerates the distance which separates them. The 

 same thing occurs with the continents. Here, for example, is the 

 Europe-Asia-Africa mass; it was united formerly to the American 

 mass, and there can still be seen on both sides of the Atlantic an 

 undeniable similarity in the outline of the shores. Between these 

 masses, which were formerly one, a fissure opened which gradually 

 enlarged, because, in the general drifting toward the west, the 

 American mass moved more rapidly than ours; and this fissure, 

 to-day from 2,000 to 6,000 kilometers in width, is the Atlantic. 

 Wegener thinks America broke away from us; perhaps we shall 

 never overtake lier. 



The chains of islands bear witness to the movement of the conti- 

 nents. The islands are comparable to the small icebergs which break 

 off on the edges from the great mountains of ice and remain behind, 

 being more retarded in the intervening waters on account of their 

 small sizes. The islands form a group of stragglers behind a con- 

 tinent which advances. Consider the insular arcs of eastern Asia, 



