352 HISTORICAL GEOLOGY 



In western North America the Silurian system, where 

 found, consists of limestone. It is in fact merely a part of a 

 thick limestone series which contains faunas characteristic 

 of the Ordovician and Devonian periods as well as of the Si- 

 lurian. The implication is plain that for long periods of time 

 the open sea held uninterrupted sway. In the region in- 

 cluding Colorado and part of Wyoming, 

 however, Silurian rocks are unknown and 

 it is possible that here was a land mass 

 in Silurian time. 



Animals of the Niagaran sea. Our 

 knowledge of the living things of the 

 Silurian is largely confined to the rich 

 and varied society of animals which in- 

 habited the clear though shallow seas of 

 the time. The Oneida conglomerate has 

 yielded no fossils, and the Medina very 

 few, perhaps because the turbulent 



FIG. 358. One of the , , , , , , . , ,. , ., 



commonest trilobites and sand-choked streams which distrib- 

 (Caiymene) of the Ni- u ted them were not attractive to 

 aquatic animals. The Niagara fauna, 

 then, may be considered by itself. 



Of the groups mentioned in discussing the Ordovician period 

 all but two made notable progress in the Silurian, the excep- 

 tions being the graptolites and the trilobites. The decline of 

 the graptolites from their position of importance in the pre- 

 ceding period was rapid. They are not numerous even in the 

 Niagara rocks, and the Devonian period witnessed their 

 complete extinction. Among the trilobites, however, the 

 descent from supremacy was more gradual. In the Silurian 

 they were still abundant, and never were they more diversi- 

 fied in form than at this time. Like the decadent nations 

 revealed to us in human history, they indulged in extrava- 

 gant and futile eccentricities, ill befitting their approaching 

 overthrow. Odd and highly ornate forms appeared i? pro- 

 fusion (Figs. 359, 360, and 361), and in most instances the 



