288 



PALEOZOIC TIME. 



stones, and 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 thick- 

 ness of less than 500 feet. 



Hence, the oscillations of level over the Interior basin were small, 

 as compared with those of the Appalachian region. Moreover, the 

 prevalence of limestone strata in the basin is evidence 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 Ocean. Through some parts of 

 the west, the Niagara and Corniferous 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 lime- 

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

 none in Bohemia — that the continent had not its one uniform inte- 

 rior 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 mounr- 

 tains, rivers must have been small. 



Life. — The expansion of the types of land-plants, insects, and Jlshes 

 especially marks the Devonian age. 



The progress of life during the Devonian is further seen in — 



(a.) The introduction of many new genera under old tribes ; for 

 example, Productus among Brachiopods, which began in America in 

 the Corniferous period, and had its maximum display, and also its 

 extinction, in the Carboniferous age ; Goniatites among Cephalopods, 

 which had its earliest American species in the Hamilton period, and 

 became extinct at the same time with Productus — a genus of inter- 

 est, as it is the first of a family (that of Ammonites) which had a 

 wonderful extension under other genera in the Reptilian age, and be- 

 came extinct at the close of that age ; Nucleocrinus (Fig. 492), an 

 early form of the Pentremites, another of the eminently Carboniferous 

 types. 



(b.) The complete or approximate extinction of tribes : as that of 

 the Cystids, which disappeared with the Oriskany period in America and 

 the Eifel limestone in Europe ; that of Favistella, Heliolites, and other 

 genera of Corals and Crinoids ; that of Atrypa, String ocephalus, and 

 other genera of Brachiopods ; that of the Chain-coral, or Halysites, 



