260 GEOGKAPHICAL CHANGES CAUSING FLUCTUATIONS [Ch. XII. 



Russian coast, 

 Greenland. 



Morton, off tlie N.W 



Former geographical changes which may have caused the 



Havi 



ng now 



fluctuations 



shown the reader that there have been endless changes in the 

 form of the earth's crnst in geological times, whereby the 

 position as well as the height and depth of the land and sea 



has 



made to vary incessantly, and that on these geo- 

 graphical conditions the temperature of the atmosphere and 

 of the ocean in any given region and at any given period 

 must mainlv depend, I shall next proceed to speculate on the 



mi 



the leading facts revealed to us by geology as explained in 

 the last two chapters. 



In order that our speculations may be confined within the 

 strict limits of analogy, 1 shall assume, 1st, That the propor- 

 tion of dry land to sea continues always the same. 2ndly, 

 That the volume of the land rising above the level of the sea 



mean 



its extreme height, is liable only to trifling variations. 3rdly, 

 That on the whole and in spite of local changes, both the 

 mean and extreme depth of the sea are invariable ; and 4thly, 

 That the grouping together of the land in continents is a 



economy 



I think it consis- 



tent with due caution to make this last assumption, because 

 it is possible that the laws which govern the subterranean 

 forces, and which act simultaneously along certain lines, 

 cannot but produce, at every epoch, continuous mountain- 

 chains ; so that the subdivision of the whole land into innu- 

 merable islands may be precluded. 



If it be objected, that the maximum of elevation of land 

 and depth of sea are probably not constant, nor the gather- 

 ing together of all the land in certain parts, nor even perhaps 

 the relative extent of land and water, I reply, that the argu- 

 ments about to be adduced will be strengthened if, in these 

 peculiarities of the surface, there be considerable deviations 

 from the present type. If, for example, all other circum- 

 stances being the same, the land is at one time more divided 

 into islands than at another, a greater uniformity of climate 





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