THE DEVONIAN AGE. 83 



them of most remarkable organization. On the 

 margins of these waters stretched vast swamps, 

 covered with a rank vegetation. 



But the period was one of powerful igneous 

 activity. Volcanoes poured out their molten rocks 

 over sea and land, and injected huge dykes of trap 

 into the newly-formed beds. The land was shaken 

 with earthquake throes, and was subject to many 

 upheavals and subsidences. Violent waves desolated 

 the coasts, throwing sand and gravel over the flats, 

 and tearing up newly-deposited beds ; and poisonous 

 exhalations, or sudden changes of level, often proved 

 fatal to immense shoals of fishes. This was the 

 time of the Lower Devonian, and it is marked, both 

 in the old world and the new, by extensive deposits 

 of sandstones and conglomerates. 



But the changes going on at the surface were only 

 symptomatic of those occurring beneath. The im- 

 mense accumulations of Silurian sediment had by 

 this time so overweighted certain portions of the 

 crust, that great quantities of aqueous sediment had 

 been pressed downward into the heated bowels of 

 the earth, and were undergoing, under an enormous 

 weight of superincumbent material, a process of bak- 

 ing and semi-fusion. This process was of course ex- 

 tremely active along tho margins of the old Silurian 

 plateaus, and led to great elevation of land, while in 

 the more central parts of the plateaus the oceanic con- 

 ditions still continued; and in the Middle Devonian, 

 in America at least, one of the most remarkable and 



