Ch. XII.] 



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might be produced, the mean temperature remaining un- 

 altered ; or if, at another era, there were mountains higher 

 than the Himalaya, these, more especially when placed in 

 high latitudes, would cause a greater excess of cold. Or, if 

 we suppose that at certain periods no chain of hills in the 

 world rose beyond the height of 10,000 feet, a greater heat 

 might then have prevailed than is compatible with the exist- 

 ence of mountains thrice that elevation. 



Since I first proposed in 1830 to account for the more 

 genial climates of former times, by showing that there is 

 now an excess of land in polar regions, Mr. Hopkins has 

 made some important calculations to prove that, by reasoning 

 on data supplied by the isothermal maps of Dove, we may 

 infer that a great alteration in climate would be brought 

 about in the northern hemisphere by what every geologist 

 must regard as slight alterations in geography. If, said he, 

 we assume ; 1st, the diversion of the Gulf-stream from its 

 present northerly course ; 2ndly, the depression of the existing 

 land of Northern and Western Europe to the amount of no 

 more than 500 feet; and 3rdly, a cold current from the 

 North, SAveeping over the submerged area, the effect would 

 be, that both on Snowdon and the lower mountains of the 

 West of Ireland the snow-line would descend to within 1,000 



feet of the sea-level, and glaciers reach the sea.* 



Now 



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everyone who is aware of the rising and sinking of land, of 

 which we have proofs since the present species of animals 

 and plants were in existence; or since the commencement of 

 the Glacial epoch, will be prepared to concede that, without 

 violating probability, we may imagine far more 

 changes to have occurred since the older Pliocene period 

 than those above suggested. Even if we admit that the 

 Glacial period began as far back as the close of the Newer 

 Pliocene era, when 5 in 100 of the mollusca were of 

 different species from those now living, we might still 



more 



as long since the older Pliocene deposits were formed, for m 

 these more than half the shells belong to extinct species. 



* Quarterly Journ. Greol. Soc. 1852. 



