34 FORMATION OF THE EARTH 



became mild or rigorous and the climate humid or dry. The 

 climate was in part also conditioned by the altitude of the 

 mountains. The Caledonian and Hercynian chains were com- 

 pletely levelled during the primary and secondary periods ; 

 but from the inclination to one another of the layers 

 constituting the two opposite faces, where the distance from one 

 another is known, of an anticlinal fold, we can compute the 

 height to which the summit of this fold was once raised, and we 

 thus discover that the ridges of these chains rose to a height 

 of many thousand metres as in the case of the present 

 Himalaya. These high mountains were covered with an eternal 

 garment of snow, as in our own time. Glaciers, of which traces 

 can be seen even in the Archaean period, moved down their 

 valleys and so cooled the air. At all epochs, therefore, there 

 have been relatively cold and relatively hot regions, and, in 

 consequence, winds and tempests, rain and snow. But all this 

 is true even of tropical regions to-day, and we shall see that a 

 tropical climate dominated the world for a long time. It is 

 tempting to attribute this to the fact, incontestable to-day, 

 that the internal heat of the earth, still considerable no doubt, 

 is more markedly felt through a solid crust where it is least 

 thick. Since the appearance of life on the earth, however, 

 this internal heat does not seem to have played any great 

 role. As a matter of fact, we possess no means of calculating 

 how much the earth's temperature rises at the centre. In the 

 deep wells of mines, where the temperature has been studied, 

 it has been found to increase steadily as one descends ; but 

 it does so in a surprisingly capricious manner. The term 

 geothermic degree has been given to the number of metres 

 corresponding to a rise of temperature of i degree. In the 

 mines of Sperenberg, which are among the deepest, the 

 geothermic degree has been measured every 200 metres to a 

 depth of 2,500 metres. It varies from 16 to 140 metres. An 

 attempt has been made to combine these observations by a 

 formula of the second degree. The formula is as follows, 

 S being the depth in metres and T the temperature (Reaumur). 



T = 7 18 + o, 0,12983572 S — o, 00000125791 S 2 



This gives us the amazing result that freezing point would be 

 reached at a depth of 3,420 metres if the temperature continued 

 to obey it at this depth. But however variable laws may be, a 



