438 A. W, GRABAU— PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



lent of the Queenston, and a younger of Siluric age, the equivalent of 

 the New York Upper Medina. The hiatus between the two is just as 

 much obscured as in the northern sections, and there is not much hope 

 of even finding the line of contact between the two. How much of the 

 red is Siluric and how much Ordovicic can probably never be ascertained, 

 but that both are represented is shown by the Upper Ordovicic fossils in 

 the basal part and the early Siluric fossils in the upper. The latter, 

 however, are the least satisfactory. 



The distribution of the Clinch is in general coextensive with that of 

 the Bays, though the latter occurs where the former has been largely or 

 wholly removed by subsequent erosion, which, however, occurred in Si- 

 luric time. In thickness the formation ranges from nothing to 500 feet, 

 the maximum occurring near the center of the area, though' disappearing 

 more abruptly southward. The present thickness is no criterion by which 

 to gage the original thickness, which was probably much greater. A 

 great part in the southern area seems to have been used up in the making 

 of the Eockwood sandstones. These latter were deposited along the bor- 

 der of a transgressing sea, so that the beds come to lie on eroded members 

 of various older formations. 



The northward and westward extension of the Clinch beds dips under 

 the younger strata of the Allegheny plateau and does not reappear again. 

 In the Nashville and Cincinnati domes these beds seem to be unrepre- 

 sented, but it must be remembered that extensive erosion occurred in 

 these regions during middle and perhaps later Siluric time. 



SUMMARY OF THE EARLY SILURIC DEPOSITS 



If we now attempt to summarize these early Siluric deposits and trace 

 the history of their formations, we are met at the outset by the fact that 

 the order of deposition is the reverse from that which we found in the 

 late Ordovicic. There we had white conglomerates and sandstones de- 

 posited first, followed by highly oxidized sands and muds which now are 

 dehydrated and red. Here we have coarser red sediments first, followed 

 by pure white sandstones or conglomerates. Siluric fossils, especially 

 Arthrophycus liarlani, seem to characterize both red and white beds. 



A further fact to be considered is that these beds as well as the under- 

 lying ones are wholly absent between the Appalachian ridges and the 

 crystalline old land, from which they must originally have been derived. 

 Furthermore, while the beds of the two series are as a rule concordant, 

 being separated only by an hiatus and disconformity, there is indication 

 at the Susquehanna Gap — though not proof — that the Tuscarora and 



