TENNESSEE AND ADJOINING DISTRICTS. 73 



doubtless facilitated the work of erosion ; and these again b}- the 

 members of the Upper and Lower Silurian groups, occupying the 

 plains and the central portions of the valleys. The Silurian strata, 

 which are thrown into numerous flexures along the valley of East 

 Tennessee, ultimately give place to others of Cambrian age as we 

 approach the Archaean Protaxis of the Unaka range, forming the 

 south-eastern margin of the plain. 



Part II. — Development of the chiee Physical Features. 



I. The Cumberland Plateau. 



The physical features, the origin of which I here propose to 

 discuss, are (1) the Cumberland Plateau, and (2) the Gorge of the 

 Tennessee River where it traverses this plateau below Chattanooga. 

 The discussion of the origin of these two leading physical features 

 necessarily involves some reference to the mode of formation of the 

 adjoining areas, that of the Valley of Eastern Tennessee on the 

 east, and that of the Silurian plain of the Cumberland River, or of 

 Xashville, on the west. An inspection of the longer diagrammatic 

 section, from the Archaean Protaxis of the Unaka Range to the 

 pl.-iin of the Cumberland River at iS'ashville, shows in order of suc- 

 cession from east to west— (1) the Unaka Range; (2) the Valley 

 of East Tennessee ; (3) the Cumberland Plateau ; (4) the Silurian 

 dome or uprise of jN^ashville ; together with the generalized stratifi- 

 cation of this tract. (See Sections, figs. 2 and 3.) 



]. The Stratification. — In dealing wdth this subject I have to 

 observe that from the base of the Cambrian beds, where they rest 

 discordantly upon those of the Archaean Protaxis, the whole series 

 of Low^er- and Upper-Palaeozoic formations succeed each other in 

 apparently conformable sequence, except at the junction of the 

 Lower- and Upper-Silurian series, where a probable discordance 

 occurs *. Throughout the prolonged period during which these 

 formations were being deposited, there was continuous subsidence, 

 with occasional pauses, over the region lying to the west of the 

 Archaean Continental area, and successive formations of marine 

 strata were laid down in vast sheets over the bed of the ocean, never 

 probably very deep. In later Carboniferous times the marine 

 deposits gave place to those of lacustrine or estuariue origin, but 

 still without any apparent discordance in the stratification ; so that 

 the Upper and Lower Carboniferons beds are apparently conform- 

 able to each other, and these again to the Devonian and Upper 

 Silurian t. 



* According to Professor J. D. Dana, this discordance is very marked in the 

 New-England States, where the Lower-Silurian beds have been inetainorphosed 

 and elevated with the Archosan rocks. In their southern prolongation this is 

 not so evident, but higlily probable. See J. D. Dana, " Areas of Continental 

 Progress," Bull. Geol. Soc. America, vol. i. 1889. 



t I use the expression "apparently conformable," because, though there may 

 be discordances of stratification, they are so small as not to have been recog- 

 nized. 



