On Geological Climate. 65 



less necessary to have recourse to astronomical causes to enable 

 us to understand them and explain them. Thus, it is now well 

 proved, that the soulevemens which have produced the principal 

 mountain-chains of our continents have no connexion with such 

 causes. In fact, the absence of all direct connexion between the 

 direction of chains of mountains, and the position of the poles 

 and the equator, indicates, with sufficient distinctness, that their 

 formation has not depended at all on the displacement of the 

 axis of the earth. In like manner, our mountains could not have 

 been the result of the shock of a comet, even though we suppose 

 comets to have a solidity which they are far from possessing ; 

 for the shock of a body in motion would be much more calcu- 

 lated to produce, in the solid external crust of the earth, inequa- 

 lities disposed more or less symmetrically round one point, than 

 ridges running parallel to one another over a large extent. The 

 other phenomena of the ancient world seem to be equally con- 

 ceivable, without our having recourse to cosmological causes 

 which are so much the less admissible as they are beyond the ordi- 

 nary murse of events. In fact, a displacement in the axis of the 

 earth, like a change in the inclination of the ecliptic, could not have 

 taken place without great revolutions in our earth, revolutions 

 which its form and its density, increasing from the surface tp 

 the centre, are far from announcing. 



Without doubt, the theory of attraction shows that the obli- 

 quity of the ecliptic experiences secular variations; but these 

 variations are confined within limits so narrow, that from them 

 no notable changes could result to our climates. Besides, it is 

 not at all in these variations that we can find an explanation of 

 the phenomenon which we are now considering. 



To understand it, it seems necessary to direct our attention 

 both to the heat and to the light which the polar regions now re- 

 ceive, and the heat and light which they received in ancient geo- 

 logical periods. 



The.se regions at present enjoy, during six months of the year, 

 a very intense light, while they experience, by turns, during the 

 other six months, the effects of twilight, or those which result 

 from the progressive enfeebling of the light. These regions, 

 then, seem little favoured in respect to the continuity of the 



vol. XIX. NO. xxxix. — .lui.Y 1835. e 



