STRATIGRAPHIC TAXONOMY 613 



which he knew contained a Saratogan fauna, while some upper bed of the 

 same held gastropods and cephalopods universall}^ accepted as post- 

 Cambrian fossils. Under the prevailing bipartite division of the Eopale- 

 ozoic the mollusks were naturally assigned to the Ordovician. 



The Saratogan age of the lower part of the Knox having been recog- 

 nized, and, as Walcott believed, that equivalent beds occurred in the 

 Mississippi Valley, in central Texas, and in the Black Hills, the first 

 and second beneath dolomitic limestones, commonl}^ referred to as corre- 

 sponding to the New York "Calciferous," it was to be expected that the 

 shales and limestones underlying the Knox and resting on the Eome in 

 east Tennessee and Alabama were, for the most part, referred by him 

 to the middle Cambrian. Only the upper division of this limestone and 

 shale sequence, the Nolichucky shale, was commonly placed in the upper 

 Cambrian.^^ 



Recent studies have practically established that the Rogersville shale, 

 the Maryville limestone, and the Nolichucky shale of the Appalachian 

 Valley section in east Tennessee, represent an epoch of great transgres- 

 sion of Cambrian seas. It is this sea — I hope it may be called the Saint 

 Croixan — that swept up the Mississippi Valley to Minnesota^^ and that 

 laid down the first Paleozoic marine sediments in Missouri, in central 

 and western Oklahoma, in central and western Texas, and in the Black 

 Hills and Big Horn Mountains of Wyoming. Whether submergent con- 

 ditions obtained in the great Cordilleran basin of the west during this 

 time is not altogether certain^ but in the Appalachian Valley the evi- 

 dence shows conclusively that the Saint Croixan is confined to the Ten- 

 nessee and Alabama basins. Belying on general faunal similarities and 

 apparent agreement in stratigraphic position, the deposits resulting from 

 this transgression have, in most instances, been referred by Walcott to 

 the late middle and upper Cambrian. Viewed from the standpoint of 

 diastrophism, I fail to see a sufficient reason for calling any part of the 

 Cambrian section in the Mississippi Valley, in Oklahoma, and in central 

 Texas middle Cambrian. On the contrary, conceding the probable high 

 average altitude of the continent during the Cambrian, the broad but 

 very shallow Saint Croixan seas make an ideal close of Cambrian sub- 

 mergent phases of American continental basins. The deposits in these 

 shallow mid-continental depressions, therefore, constitute a perfectly 



«».Tournal Geology, vol. xi, 1908, p. 310. 



^ Besides a number of unclassified trilobites the following brachiopods are common 

 to the upper Cambrian Tonasauga shale in the Appalachian Valley and the Saint Croixan 

 deposits in the Mississippi Valley : OhoJtis lamhoniei. O. Mnw, Westonia eUa, TjitujuleUa 

 similis, L. (Jeni(Jer(ita, and Liii(ji(Jei>sis aciiiiiiiiatd. A variety of the last is found also in 

 the Ozarkian in both provinces. 



