THE REVOLUTIONS OF THE CRUST OF THE ExVRTH. 311 



of their form, are destroyed, carried far from the iiioutb, and deposited 

 anew under the form of strata. The clay and sand likewise deposited 

 acquire in time consistency, and mixed and cemented by means of solu- 

 ble materials in the water are sometimes converted into hard rock. At 

 other times, however, by the constant action of the waters, the sands are 

 washed upon the low shores, where they are quickly dried and carried 

 by the winds upon the continents. The hillocks or dunes which mark 

 the progress of the sand invade many fertile countries, and threaten to 

 convert them into arid deserts. The coasts of Guyenne (Lande) are an 

 example ; the sand-banks have there made great progress, although they 

 increase at the rate of not more than two meters a year. 



If the desert of Sahara was formed in this way, the sand-banks without 

 doubt commenced at the western side of the African continent, where 

 it still continues. But in order to reach from the Atlantic to the shores 

 of the Nile (and it is not even two thousand years since this has taken 

 place,) the moving sands must have taken more than two millions of 

 years, and the distance which they have passed over is about 450 myria- 

 meters (2,800 miles). 



The fluid envelope of the earth is not only affected by the sun ; the 

 moon also exercises an influence by its i)roximity in producing oceanic 

 and atmospheric tides.* The tides of the fluid envelope, in general, 

 are the i)roduct of the combined -movement of the earth around its axis 

 and around the center of gravity common to the moon and to the earth 

 situated in the mass of the terrestrial globe. Eefore arriving at this 

 explanation, t different theories were projiosed, generally incapable of 

 explaining the periodical return of the waters, when this could not be 

 referred to the attraction of the moon.^ 



The circulation of the fluids at the surface of the earth depends, as 

 we see, upon the attraction of the moon and of the sun, and upon the 

 radiant heat of the latter which produces the evaporation of the waters 

 and their condensation in the form of rain, dew, hail, snow, &c. The 

 influence of the nucleus of the earth is less and is manifested only by 

 the changes produced in the interior of the crust and at the surface, 

 consequent upon risings, eruptions, and earthquakes. The heat acts 

 again upon the waters, which penetrate the crust of the earth, raising 

 the temperature according to the depth. Hence the origin of many 

 thermal springs. In order to explain the number of these springs, 

 many scientists have generalized the action of the central heat upon 

 the waters which penetrate the crust by fissures or by infiltration. 

 They have thought that the waters are distilled by contact with the 

 incandescent mass, and, deprived of all saltness, are condensed in the 

 clefts of the rocks whence they gush out at the surface of the earth. 



*C. M. Elliott, On the Lunar AtmosplieriG Tide at Singapore, Phil. Trans, of the Roy_ 

 Soc. of London, 1852, 1st part, p. 125. 

 t Croll, Phil. Mag., April, 1864, p. 287. 

 X Cams, Briefe ubcr das Erdenlchcn, p. 153. 



