GENERAL OBSERVATIONS. 301 



nently dry, would have been arranged anew by the waves in times 

 of submergence. 



3. Geographical changes. — The history of the periods of the Devo- 

 nian has been shown to be, like that of the Silurian periods, a history 

 of successive oscillations in the continental level, — the position of 

 the accumulating deposits varying more to the east or to the west 

 with the varying location of the subsiding or emerging areas. 

 Throughout the whole, the Appalachian region continued to be 

 well denned. Its deposits consisted mainly of shales and sand- 

 stones, and they have a total thickness of not less than 15,000 

 feet ; while in the West the rocks are for the most part limestones, 

 with a thickness of less than 500 feet. 



It appears, moreover, that in the Devonian as well as Silurian 

 age, the interior region was covered with but thin beds of any 

 kind, — thin sandstones when sandstones were formed, and thin 

 limestones compared with the cotemporaneous shales and sand- 

 stone strata farther east ; and hence the oscillations of level there 

 indicated are small as compared with those of the Appalachian 

 region. Moreover, from the prevalence of limestone strata in the 

 "West we learn that the great mediterranean sea of the Silurian age 

 was continued far into the Devonian, opening south into the 

 Atlantic and Gulf of Mexico, and reaching north probably to the 

 Arctic. Through some parts of the West, the Niagara and Upper 

 Helderberg limestones — the formations of that interior sea — follow 

 each other with but little interruption. 



European Geography. — The European continent in the Devonian 

 age could not have had the simplicity of features and movement 

 that characterized the American. It is obvious from the great 

 diversity of the Devonian rocks — sandstones at one end of Britain 

 and limestones at the other, limestones in the Eifel on the Rhine 

 and almost none in Bohemia — that the continent had not its one 

 uniform interior sea like North America, but was an archipelago, 

 diversified in its movements and progress. 



There may have been proportionally more elevated heights over 

 the area ; but it is still true that there was little of it dry ; that the 

 loftier mountains had not been made, — the Alps and Pyrenees being 

 hardly yet in embryo ; and that, with small lands and small moun- 

 tains, rivers must have been small. No fresh-water deposits have 

 yet been distinguished ; the salt ocean was nearly universal. Within 

 it, animals were living, and the continent was forming at shallow 

 depths. 



Life. — The introduction of land-plants and that of fishes are the 

 two great steps of progress that especially mark the Devonian age. 



