350 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



can be admitted without difficulty. As to the unequal temperature of 

 the two hemispheres, we must cease to attribute it to the unequal distri- 

 bution of the waters ; for the earth is undoubtedly in the best condition 

 to retain its heat when there is a preponderance of the sea, as conti- 

 nents lose more heat by radiation. 



The hypothetical character of a mass of ice so considerable, disap- 

 pears, if we take into account the effects of the ancient cupola of ice 

 which covered all Lapland, Northern Russia, Iceland, a part of North 

 America, and Siberia. Some geologists, among others Professor Agas- 

 siz, affirm that a cupola of ice covered, during the glacial period, the 

 largest part of our hemisphere, and that the erratic boulders of Central 

 Europe are only the moraines of this immense glacier, which would 

 justify in a certain point of view the celebrated remark of Humboldt, 

 that the earth is formed by the union of two mountains by their base. 



The objections made to the hypotheses of M. Adh6mar cannot lessen 

 its scientific value. Far from sharing the fate of many theories pro- 

 posed to explain the great geological cold periods, it on the contrary 

 gains more and more upon the minds of those who attentively examine 

 the circumstances of the production of these phenomena. But while 

 acknowledging its merits, the necessity of certain modifications should 

 be recognized. Thus we may admit that the displacement of the center 

 of gravity of the earth takes place every 10,500 years ; but, as the pres- 

 ent eccentricity of the earth's orbit is not great, the changes of tempera- 

 ture of the two hemispheres is not sufficient to produce great accumu- 

 lations of ice upon one of the poles. 



The configuration of the continents cannot then change much by the 

 removal of the waters from one hemisphere to the other. It is only 

 great eccentricities of orbit which can produce a general cooling of a 

 hemisphere and accumulation of polar ice sufficient to produce any con- 

 siderable displacement of the waters. Then the oceans invade the conti- 

 nents and cover them to heights corresponding to the lines of erosion 

 observed by M. Sharps. 



The savans who have investigated the action of ice during the Silu- • 

 rian formations of the old red sandstone of the Cretaceous period, &c., 

 have at the same time established the consecutive existence of condi- 

 tions very favorable to the development of organic life. The characters 

 of the plants and of the animals indicate a climate warm and uniform, 

 which contrasts strangely with the severity of the climate of the glacial 

 periods. Moreover, this uniform climate did not reign only in the tem- 

 perate regions ; even in the more elevated latitudes its influence pre- 

 vailed. Of this the coal-beds of the island of Disco, alternating with 

 deposits of clay, the submarine forests of the coast of Greenland, the 

 fossil remains of vegetation found in Spitzbergen and upon several ice- 

 covered islands of North America, are sufficient proof. 



In the instances mentioned, we cannot suppose that the vegetable 

 debris were transported by marine currents, nor that the combustible 



