226 PALEOZOIC TIME — LOWER SILURIAN. 



extinct. This singular type of Crinoids had its climax in the 

 Lower Silurian, though not its final extinction. After this its 

 species were few ; while there is a great increase of Crinideans. 



The number of Lower Silurian species known to have become 

 extinct in Great Britain is about 600, and in Bohemia, according to 

 Barrande, 300. 



Thus far in American Geology, no evidence has been detected 

 of (1) fresh-water lakes or deposits, or (2) of land or fresh-water 

 life. The living species had been mainly Molluscan and Eadiate, 

 because these are the water-types in the Animal kingdom. And 

 with them were associated the water-divisions of Articulates, — 

 Worms and Crustaceans ; but not yet the water-division of 

 Vertebrates, — Fishes. The world had been populated solely by 

 Radiates, Mollusks, and Water-articulates. The continent was, 

 like its species, submarine in its mode of existence. It was already 

 outlined, and in its heavings and progressing changes its coming 

 features were shadowed forth, — even the Appalachian chain, and 

 the great lakes, — although the mountains had not yet a foot of 

 their present height above the seas, nor the lakes more than the 

 beginnings of their depressions. 



DISTURBANCES CLOSING THE LOWER SILURIAN ERA. 



The strata of the Lower Silurian in North America appear to 

 have been spread out over the Interior Continental basin in hori- 

 zontal beds of great extent, and to have followed one another 

 without much disturbance of the formations, — that is, no upturn- 

 ing that exposed outcropping edges to be overlaid by later hori- 

 zontal deposits, producing thereby unconformability. In the Potsdam 

 period there were local eruptions of trap in the region of Lake 

 Superior; hut no positive evidence of dislocations during the 

 Lower Silurian era is yet known. The period that witnessed the 

 gradual accumulation along the Appalachian region of 15,000 feet 

 of deposits, we are sure, was long; and the more remarkable, there- 

 fore, is this exemption from catastrophe. Yet there is no doubt of 

 extended oscillations in the water-level over the continental area. 

 The rocks have afforded clear evidence of an indefinite range of 

 shallow waters and sand-flats throughout the great interior in the 

 Potsdam period ; of somewhat deeper waters over the same wide 

 area in the Trenton period, and such a uniformity of moderate 

 depth as admitted of the formation of limestones of continental 

 extent ; then of other changes of level in the Hudson period, 



