50 FORMATION OF THE EARTH 



with the Permian Period. It probably then gave rise to 

 phenomena analogous to those which have produced the 

 Gobi desert at the foot of the Tibetan massif. 



We now approach the beginning of the Secondary Period, 

 the Trias, during which, it appears, the height of the Hercynian 

 mountain chain had been considerably reduced, either through 

 long-continued erosion or local subsidences. The mountains, 

 which play so great a role in atmospheric condensation, having 

 been markedly levelled, glaciers seem to have disappeared and 

 the rains to have become less abundant. The dry and hot 

 climate of certain regions during the Permian epoch appears 

 to have become general, or, at least, more extended. 



Although local differences in the mean temperature existed, 

 nothing indicates that at this period there were climatic zones 

 comparable to those existing now. Until positive proof is 

 forthcoming that, at this epoch, the earth's axis approached 

 a position normal to the ecliptic, with which periodic variations 

 of position are inconsistent, and also that at that time its orbit 

 approximated to that of a circle, we must assume, as has been 

 pointed out, that the apparent diameter of the sun was very 

 much greater than it is to-day. A very slight decrease in this 

 diameter, without the axis of the earth changing its position, 

 would be sufficient to cause the appearance of polar zones, 

 separated by a torrid zone that was, in fact, very extensive, 

 for there still existed at this period reefs and islands formed of 

 madreporic coral in certain areas of Europe corresponding to 

 Alsace, the north of France, and Wales. This torrid zone seems 

 to have been characteristic of the climate of the Jurassic 

 Period. 



Throughout this quiescent Secondary Period, during which 

 not a single chain of mountains was raised, and in which the 

 slow action of erosion continued, nothing could have been 

 produced suddenly — neither glaciers in the deep valleys nor 

 violent atmospheric condensations ; but a slow retreat of 

 coral formations towards the south indicates a gradual 

 restriction of the torrid zone and a compensating appearance of 

 temperate zones towards the polar regions. 



Corals long persisted in Alsace, in Switzerland, in the cantons 

 of Argovie and Fribourg, and in the Jura, whilst in Lorraine 

 they formed reefs twenty metres in thickness. The flora 



