738 DYNAMICAL GEOLOGY. 



eras. — This cause is so far within the region of the hypothetical as 

 hardly to merit consideration until all others admitting of investi- 

 gation have been proved insufficient. 



The first of these causes is beyond doubt a real one ; yet its 

 effects must have been too small, especially after the Carboniferous 

 age, to have produced the whole amount of change which took 

 place. 



The second is also of undoubted value, as explained on page 45. 

 To appreciate its influence, it is necessary to suppose the existing 

 globe with its climates changed in its lands. For example, to pro- 

 duce the climate of the Coal period, the amount of land should be 

 reduced one-half in area, far the larger part of this be brought 

 nearly to the ocean's level, all the highest mountains be lowered 

 or levelled, and the greatest elevations made not to exceed 6000 or 

 8000 feet, and then to be of comparatively limited extent. 



The Polar lands were in existence, as the coal fields show, but 

 may have been comparatively small. Facts seem to prove that 

 such changes might reduce the higher latitudes to the condition 

 of Fuegia. But Fuegia has alpine plants within one thousand feet 

 of the sea, and a mean temperature but little above freezing. The 

 result is still not what the facts require ; for there is no reason to 

 believe that there was any alpine or sub-frigid vegetation at Melville 

 Island, or that the plants differed essentially from those of Penn- 

 sylvania. This warm climate of the poles was hardly less striking 

 in the middle Mesozoic. For, while Keptiles are especially charac- 

 teristic of the tropics, there were Ichthyosaurs and Teleosaurs in 

 the Arctic. Sir Edward Belcher found an Ichthyosaur on Exmouth 

 Island, in latitude 77° 16' N. and longitude 96° W., 570 feet above 

 the present sea-level; and Captain Sherard Osborn found two bones 

 of a species allied to the Teleosaur on Bathurst Island, in latitude 

 76° 22' N. and longitude 104° W. 



The third cause mentioned has unquestionably acted in the 

 earth's history, if the globe was once in igneous fusion. But it 

 may well be questioned whether by the commencement of the 

 Silurian age the crust had not attained a thickness which would 

 have rendered the internal fires no longer a source of heat for the 

 earth's surface. That the heat issuing through the crust was so 

 great at the poles in the Carboniferous age as to raise the mean 

 temperature, by radiation into the atmosphere, from 40° F. to 60° F. 

 (the climate probably required for the vegetation of the era), is not 

 true ; for the same amount of change in the temperate and tro- 

 pical zones would have rendered them uninhabitable by most plants 

 and animals. But some few degrees of heat may have been received 



