362 E. O. IJLRICH REVISION OF THE PALEOZOIC SYSTEMS 



it, the evidence is altogether indicative of very shallow seas. Other evi- 

 dence leading to the same inference is found in the frequent lateral and 

 vertical oscillations of the continental seas, again in the presence within 

 the area of the Appalachian Valley of narrow lands positively shown by 

 physical criteria to have been, as a rule, of low relief, and finally in the 

 wide distribution of bottom-dwelling species and faunas, whose dispersion 

 across great depths would have been practically impossible. Deep wells 

 have repeatedly proved the existence of the same faunas, apparently no 

 less well developed and abundant, liundreds of miles out from the known 

 shores of the sea in which they lived. A very convincing instance was re- 

 cently afforded by a middle Eden fauna of thirty species taken about 

 2,000 feet beneath the surface at Waverly, Ohio. 



Depending on such evidence, I have come to entertain the belief that 

 the average depths of the Paleozoic continental seas were less than 300 

 feet, and that none attained depths exceeding 100 fathoms. This maxi- 

 mum depth possibly was exceeded locally in some of the intramarginal 

 troughs, like the Levis channel, which Euedemann thinks may have been 

 much deeper. However, the very common occurrence of thin beds of 

 limestone conglomerate in the graptolite-bearing sediments of these 

 troughs seems to militate very strongly against the assumption of great 

 depths. 



Effects of currents on deposition — Improbability of marine scour. — 

 Some geologists, notably Willis,-^ endeavor to account for known local ab- 

 sence of sediments of certain ages in areas of continental seas by assuming 

 "non-deposition and even scouring of bottoms . . . where they are 

 swept by currents whose load is less than their efficiency." This view is 

 based on such well known recent instances as (1) the reputed scour of 

 the Gulf Stream where it flows across the marginal platform of the con- 

 tinent between Florida on the one side and the Bahamas and Cuba on 

 the other; and (2), the stony, cleanly swept, submarine ridge which 

 stretches between Scotland and the Faroe Islands and separates the north 

 Atlantic and Arctic basins. Willis admits that non-deposition and 

 marine scour is today, and has always been, an exceptional condition, 

 "restricted to comparatively shallow waters, in the path of a relatively 

 strong marine current." But, he goes on to say, "the epicontinental seas 

 of the periods of great marine transgressions (Cambrian, Ordovician, 

 Silurian, Devonian, Mississippian, and Cretaceous of North America, for 

 instance), opened channels across the continent, through which oceanic 

 currents circulated as the Gulf Stream flows from the Caribbean to the 



Bailey Willis : Science, vol. xxxi, Feb. 18, 1910, p. 249. 



