CONSEQUENCES OF CENTRAL HEAT. 



145 



of years, the surface of the earth must have passed through every 

 degree of heat, from the state of fusion in which the centre still is 

 to its present degree of cold ; consequently, there was a time when 

 the temperature of the earth was such as to do away with differences 

 of climate, or an atmosphere of vapour, which, by overcoming radia- 

 tion, diminished the rigour of winter. Then vegetation, and life 

 generally, could be as equally maintained in all latitudes as in a 

 hot-house. From this it follows, that plants and animals now found 

 only between the tropics could then live anywhere, even under the 

 poles, which were not then encumbered in ice. It is therefore not 

 astonishing that we should find the remains of these various creatures 

 buried nearly on the spot where they lived, in countries which are 

 now the coldest in the world, and in which it would be impossible 

 for them to live at the present time. 



There is in England, on the island of Portland, and at several places on 

 the continent, intercalated in other deposits, a bed of black matter, called 

 dirt-bed, and small argilla'ceous beds, in which, among- a great many vege- 

 table remains, bedded and scattered, are various plants in their place of 

 growth (Jig- 225), the roots of which extend into the fissures of the calca- 

 reous soil beneath. There- 

 fore, there must have been a 

 vegetable soil, on which all 

 the plants now buried in the 

 earth then grew. But all 

 the species found in this bed 

 belong to genera, such as 

 cycas and zamia, which 

 now live only in the tropics, 

 and the remains of animals 



Fi<r. 225. Portland dirt-bed. 



also belonged to the same zone; consequently the mean temperature at the 

 time of this formation was very different from what it is now in England. 



Most of the coal deposits of Europe lead to a similar conclusion. Entire 

 trees with their roots, many of them still erect, are found, as in the mine of 

 Treuil, near St. Eticnne (Jig. 226), in the mines of Anzin (North) in Eng- 

 land, in Scotland, &c., which seem to indicate, as in peat- bogs, plants that 

 grew very near the places where they are now found. It is evident from 

 the perfect preservation of the most delicate parts of plants, the manner in 

 which the leaves are extended on schists, that these remains could not have 

 *>cen carried far. All the remains of plants found in these deposita belong 

 .o the equisita'ceae, lofty ferns, to the lycopodea'ceae, &c., and cannot be 

 compared with those now existing in the tropics; consequently, the climate 

 of Europe must have been then very different from what it is at present. 



We find, in the latitudes of Europe, certain beds containing the remains 

 of intertropical plants, but we also find above them considerable deposits in 

 which are dicotyle'donous plants of the present time. The formation of the 

 1 st deposits, then, must have taken place long after the first; and it is pro- 

 bable that between the epochs, a period of time elapsed, sufficient for cooling 

 the surface of our planet. 



Madrepores .of reefs, which now do not exist beyond the tropics, then evi 

 dently extended to the polar circle. In fact, the limestones of different 

 periods contain a great number, and frequently show that reefs existed com- 

 parable to those of our days. Facts show that the limits of these banks of 

 ao'ophytes have retrograded, from the period of the deposit of the oldest 

 13 



