PALEOZOIC TIME — DEVONIAN. 629 



period. With the variations in the fineness, or other characteristics of the 

 beds, as H. S. Williams has illustrated, the species vary. The fine shales of 

 the Marcellus and Genesee shales have few and small species, owing to 

 some unfavorable conditions ; and, in part, the species are repeated in each 

 later return of the beds to fine shales. With the coarser sand-beds of the 

 Hamilton and Chemung, life abounds ; but Brachiopods and Lamellibranchs 

 predominate, especially in the latter, where Trilobites fail completely. With 

 beds of intermediate character, as those of the Portage, life is much less 

 abundant than in the Chemung — except at one time of change to beds 

 allied to those of the Hamilton and Chemung (the Ithaca beds), when the 

 life takes a character resembling that of the latter period. A thin lime- 

 stone stratum in some cases indicates by the species an approximation again 

 to the clearer waters of the Corniferous. There are thus alternations in 

 living species correlatively with alternations in kinds of deposits. The 

 species evidently migrated in the direction in which the conditions were 

 favorable to them. The faunas of each stratum are not strictly faunas of 

 epochs or periods of time, but local topographical faunas. After the Cor- 

 niferous period, Corals, Crinoids, and Trilobites still flourished somewhere^ 

 as before ; but they are absent from the Central Interior until the Carbo- 

 niferous age opens. 



The condition producing the Genesee shale in New York appears to have 

 spread westward over Ohio, and to have invaded the Central Interior through 

 Michigan, Indiana, the southern half of Illinois, and southward to Tennessee; 

 and to have continued to prevail over this great region through the remainder 

 of the Devonian era with but little change. The area was mud-making, with 

 more evidence of fresh-water or brackish-water life than of marine conditions, 

 and it probably had its extensive shallow lagoons and bayous in which lived 

 the great Ganoids and Eurypterids. During the Later Devonian, in the 

 Eastern Interior Sea, the Catskill sandstone to the northeast — a shore and 

 off-shore formation of the Interior Continental Sea — reached a thickness of 

 3000 to 7545 feet (I. C. White), because it lay within the range of the Appa- 

 lachian geosyncline. 



If the condition of the Atlantic border, its sounds and bays, with their 

 varying depths and fortunes, and of off-shore deeper waters and depositions 

 and fresh-water inlets, be taken as a type of the conditions and depositions 

 that existed in several successions within the Eastern Interior Sea, no 

 difficulty will be found in finding a reason for all the variations in wave 

 action, in tidal and current action, in depth, in purity of waters — ranging 

 off to over-fresh or over-salt conditions, which may be needed to explain 

 the geological and biological facts of the Middle and Later Devonian. 



The effects of tidal currents appear to be marked in the Chemung beds 

 of western iSTew York and Pennsylvania, and eastern Ohio. The strata of 

 coarse conglomerates occurring among the sand-beds appear to be due to their 

 action. The tidal waters, which, in their rounds, converged from the south 

 and west toward the head of the Eastern Interior Bay, with increasing height 



