THE DRIFTING OF THE CONTINENTS TERMIER 233 



even transformed — another hypothesis — into an extremely mobile 

 liquid. Let us consider the passage from the solid to the liquid state, 

 by the rise of the isogeothermes ; it is accompanied by great expan- 

 sions of the basalt and a strong diminution of its densit3^ The bot- 

 tom of the seas are elevated ; consequently as the mean level of the seas 

 rise also, the continents would follow this centrifugal movement; 

 but they are retarded because, being immersed in a liquid the density 

 of which diminishes, they tend to sink deeper. The result is an 

 encroachment of the sea on the continent, which the geologists call a 

 transgression or a positive vertical movement. Some millions of 

 years pass; the radioactivity becomes quiescent; all the phenomena 

 are reversed; the sea recedes; it is a retrogression, a vertical negative 

 movement. At will we shall thus make marine transgressions and 

 retrogressions, constant or intermittent, and we shall give them any 

 extent that we wish ; we could as well make them universal. In this 

 way we shall account for the vertical oscillatory movements. 



To explain the tangential movement and the possible drifting of 

 the continents, Joly invokes the tides. In the periods "of revolu- 

 tion," as he calls them, when the basaltic envelope is become molten 

 and very liquid, very mobile, nearly to the surface, this liquid, 

 mobile and yet very heavy, undergoes, from the attraction of the 

 sun and the moon, very strong tidal movements, much stronger, in- 

 deed, than those of the sea, tides which tend to retard the speed of 

 rotation about the axis of the superficial terrestrial zones. This re- 

 tardation, which decreases quickly in depth on account of the in- 

 creasing viscosity — another hypothesis — is very marked in the parts 

 of the liquid pyrosphere which are immediately beneath the bot- 

 tom of the oceans; naturally so much the more marked the nearer 

 the Equator. Thus originates the tangential force, proceeding 

 from east to west, in a sense inverse to the rotation, the tangential 

 force of the molten magma on the submerged parts of the continen- 

 tal masses. Thus originates the drifting of the continents, and, as 

 the continents are very fragile — another hypothesis — they break and 

 fold easily and in this manner the mountain chains are formed. To 

 those who would object that the theory thus presented explains 

 neither the transverse chains parallel to the Equator, as the Alps 

 and the Himalayas, nor the chains near the poles, I imagine that 

 Joly woidd respond that the poles in the course of time have changed 

 places in relation to the great continental masses, and that the 

 Tethys, on whose site to-day the Himalayas and the Alps run parallel 

 to the Equator, was perhaps formerly an ocean extending from north 

 to south, as our Atlantic. The speaker would answer that the chains 

 which actually extended along the shores of the Pacific were very 

 nearly of the same age as the Alpine chain and the Himalayas, and 

 that the difficulty of explaining the simultaneous origin of all the 



