THE CENTRAL BASIN OF TENNESSEE. 81 



taceous rock runs into the shore for neai-ly 1 miles. This represents 

 a break in the older formations and is apparently an old river channel 

 lying between the courses of the modern Tennessee and Big Sandy 

 rivei'S. In Decatur County there is another old bay much broader 

 than the Benton one but not so long. In both of these bays the head 

 lies to the north-east and the opening into the ocean to the south- 

 ■west. This is not the direction of the dip of the country which in this 

 part of the State is to the north-west or almost at right angles to the 

 course pursued by the streams which hewed out the old channels 

 mentioned. Streams, however, do not always follow the dip of the 

 beds through which they may have to cut their way, but in most in- 

 stances they follow the general slope of the country. If then these 

 channels indicate the general slopes of the countiy during the Upper 

 Silurian period, a combination of oscillations must have taken place 

 to arrive at the present state of affairs. 



This old shore line indicates a continuance of oceanic conditions 

 over all the area of the States of Tennessee and Kentucky, west of 

 the Tennessee River up to as late a date as the Cretaceous period, and 

 whatever changes the region east of this line may have undergone, 

 none appear to have interrupted the continuity of the sea's occupation 

 throughout Western Tennessee or the Mississippi Valley. 



If we turn now to the eastern side of the basin we will find the 

 Highland Rim existing as a broad tract of country extending from the 

 €dge of the escarpment back to the Cumberland Tableland, and occu- 

 pied by rocks belonging to the lower or sub-carboniferous group, 

 the Protean beds occupying the territory immediately adjacent to the 

 basin, and the mountain limestone lying close to and passing under 

 the carboniferous beds forming the tableland. The same sub-carbon- 

 iferous beds form the surface rock of a great part of the State of 

 Kentucky lying immediately to the north. 



The elevation of the Highland Rim has already been given as 

 between 950 and 1000 feet above sea level. From this plateau the 

 Cumberland tableland rises almost precipitously to a vertical height of 

 over 1000 feet or 2000 feet above the level of the sea in Mobile Bay. 



