CHAPTER V. 



THE DEVONIAN AGE. 



PARADOXICAL as it may appear, this period of geological 

 history has been held as of little account, and has even 

 been by some geologists regarded as scarcely a distinct 

 age, just because it was one of the most striking and 

 important of the whole. The Devonian was an age of 

 change and transition, in both physical and organic 

 existence ; and an age which introduced, in the 

 Northern hemisphere at least, more varied conditions 

 of land and water and climate than had previously 

 existed. Hence, over large areas of our continents, 

 its deposits are irregular and locally diverse; and 

 the duration and importance of the period are to be 

 measured rather by the changes and alterations of 

 previous formations, and the ejection of masses of 

 molten rock from beneath, than by a series of fossil- 

 iferous deposits. Nevertheless, in some regions in 

 North America and Eastern Europe, the formations of 

 this era are of vast extent and volume, those of North 

 America being estimated at the enormous thickness of 

 15,000 feet, while they are spread over areas of almost 

 continental breadth. 



At the close of the Upper Silurian, the vast con- 

 tinental plateaus of the northern hemisphere were 

 almost wholly submerged. No previous marine lime- 



5 ?* 



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