EXTENT OF PALEOZOIC CONTINENTAL SEAS 307 



limestone. Both the limestone and the shale are commonly reddish in 

 color. The fossils, too, often exhibit evidence of wear on a beach or in 

 very shallow water. 



As to the beds succeeding this late Eichmond horizon, they vary in 

 age from early Niagaran to Mississippian. The latter sequence occurs 

 in the Big Horn Mountains, and is perhaps more common in the far 

 western localities than the Silurian succession. A fact of great signifi- 

 cance is that this late Eichmond bed has been recognized in every detailed 

 section containing middle Ordovician rocks in the far west that has been 

 brought to my attention. 



SLIGHT EROSION OF INTERIOR SILURIAN AND DEVONIAN LANDS 



The succeeding Silurian and early Devonian rocks of west middle Ten- 

 nessee present similar evidence of slight erosion of adjacent contempo- 

 rary lands. The N'ashville dome seems to have been an island of consider- 

 able size during this time, its eastern shore extending beyond the present 

 outcrop of Ordovician rocks. During the Niagaran its western part 

 subsided gently, but not continuously, the advancing sea leaving a locally 

 visible record in the overlapping sediments. Following the Louisville 

 limestone, which is late Niagaran, the sea retreated, the final Silurian 

 deposits being more local in occurrence than the earlier beds of the 

 period. These late Silurian deposits in west Tennessee, like those in 

 the Cumberland basin in Maryland, were marine, while apparently con- 

 temporaneous Cayugan deposits of more interior areas (western New 

 York, Ohio, Michigan) were in part at least non-marine. 



With the beginning of the Devonian the sea began once more to ad- 

 vance. This is shown by overlapping Helderbergian deposits. The 

 edges of these Silurian and early Devonian overlaps are so well preserved 

 that their thickness may be measured in places by inches. Thin wedges 

 of late Onondaga follow the Helderbergian at only a few points, the usual 

 succession on the west and south sides of the Nashville dome being Chat- 

 tanooga shale on Helderbergian or older beds. Most of the exposed 

 Ordovician, Silurian, and early Devonian areas in Tennessee appear, 

 therefore, to have been raised above the plane of marine sedimentation 

 during the Onondaga and such considerable parts of the time scale as 

 intervened between the Onondaga and the local representative of the 

 Chattanooga shale. The middle Tennessee representative of this shale, 

 including the Hardin sandstone, I regard as corresponding somewhat 

 imperfectly to the Cleveland shale, the Bedford shale, the Berea grit, 

 and the Sunbury shale of the Ohio section. 



The Chattanooga shale, which transgresses the edges of the preceding 



