﻿NO. I SMITHSONIAN EXPLORATIONS, I925 II 



tion of interesting structural features indicating discordant relations 

 and areal restrictions in rock formations which had hitherto been 

 thought to be essentially horizontal, widespread, and conformable 

 with each other. 



At various times during the Paleozoic era, the area now termed 

 the Central Basin was uplifted above the sea as a low dome — the 

 Nashville Dome, as it is generally known — the rocks sloping gently 

 away on all sides. At times this dome was submerged partially 

 or entirely, the deposits remaining of those then formed showing 

 the extent of such submergences. At other times, the sea invaded 

 the area only in bay-like indentations in which rocks of various 

 interesting types were laid down. The resulting rock deposits hold 

 the story of these various invasions of the Nashville Dome, but the 

 strata, mainly limestone and shale, are so much alike that detailed 

 preliminary studies had to be undertaken before the formations 

 and their extent could be satisfactorily discriminated and mapped. 

 From such studies in previous seasons it was determined, for ex- 

 ample, that the Bigby limestone of Middle Ordovician age, the 

 source of much of the Tennessee brown phosphate, was deposited 

 in an arm of the sea which covered only the southwestern and 

 western parts of the Nashville Dome. The succeeding massive Cannon 

 limestone was developed, on the contrary, in a broad embayment 

 occupying the eastern two-thirds of the dome, in which it varies 

 in thickness from an inch or two along the ancient western shore 

 line to several hundred feet along the eastern side of the basin. 

 The Early Silurian, Richmond formation, also proved to have been 

 formed in similar but much narrower bays entering the dome on 

 the northern, western, and southern sides. The relative narrow- 

 ness of these embayments as contrasted with the preceding Bigby 

 and Cannon limestone bays indicates decided warping or wrinkling 

 of the surface during the transition from the Ordovician to the 

 Silurian. These restricted Richmond areas are also notable for 

 their iron ores and marble deposits. The later Silurian, Devonian, 

 and Early Mississippian formations, in many cases, were deposited 

 in similarly restricted areas. 



The occurrence and thickness of an Early Mississippian forma- 

 tion accumulated in one of these ancient narrow bays is illustrated 

 in figure 14, showing the New Providence shale, a celebrated crinoid- 

 bearing formation. Here, however, another structural feature is 

 shown in that the horizontal strata of the New Providence shale 

 are obliquely cut across by rocks of the overlying formation of 



