290 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



cepted that the Coal period required a temperature of from 27^ to 28^ 

 C, and that the temperature of our day is from 7° to 15^ at the mean. 

 Gibbert has calculated the duration of the same period to be five mil- 

 lions of years. But why admit a temperature of 27^ for the carbonifer- 

 ous period ? The preponderance alone of the Crypiogames over the Phan- 

 erogames does not justify this estimate, as proves the flora of Kew 

 Zealand, which is almost exclusively cryptogamic. Moreover the cool- 

 ing of a mass of basalt takes place in a medium and under conditions 

 quite different from those which determine the cooling of the earth. It 

 is, then, with little hope of attaining the truth that we try to calculate 

 the duration of geological ages. We must, however, admit that they 

 were immense compared with our short existence, and in every case 

 greater than the figures mentioned. 



CHAPTEE II. 



CENTRAL HEAT. 



The central heat is a constant source of force?, which, in difl'erent 

 ways, act upon the crust of the globe. 



The contraction of the solid crust, the cracking of this crust, and the 

 escape of incandescent matter are direct consequences of the loss of 

 this heat. The plications, the faults, the elevations and depressions of 

 the sedimentary crust take place because the sediments can no more 

 contract, but, on account of their weight, sink at certain points of the 

 solid nucleus. 



From these two kinds of forces there result a series of phenomena, 

 of which the rising and sinking of entire countries, earthquakes, and 

 volcanic eruptions are the most important. 



The actuality of these risings and sinkings of the crust of the globe 

 has been confirmed in so many ways that all savans now agree in attrib- 

 uting to them a large part in the successive modifications of the surface 

 of the earthy and in the raising of the continents. If there were no 

 other convincing proof, the fact that the mountains are, on an average, 

 10,000 meters above the mean of the oceanic depressions would confirm 

 the idea that these masses cannot be of marine formation, but are the 

 product of a central force — of an action slow but constant, rarely sud- 

 den or violent. 



Upheavals and subsidences are observable at many points of the 

 globe, and some even are taking place under our eyes, but so slowly 

 that the effect is perceptible only after a long period of action. Thus it 

 is i>roved that the northern part of Sweden is rising so rapidly that we 

 may foresee tlie time when the Gulf of Bothnia will be completely dry. 

 This elevation diminishes toward the southern part of this country, and 

 there is even a siukiu g of the shores of Germany, of the islands of 



