THE CENTRAL BASIN OF TENNESSEE. 75 



In addition to the serrated or j-ather fringed like appearance of tlie 

 ■■margin of the rim, or the escarpment, there is yet another peculiarity 

 ■connected with it — that is, the absence of any terracing npon this 

 escarpment. It has been assumed that the basin was at some former 

 time the bed of a lake. In no place has it any terrace markings 

 indicating that the basin was at any time the seat of an ancient lake. 

 The northern and western divisions of the margin show appearances 

 •somewhat terrace-like, but these divisions are, as we shall afterwards 

 see, due to the constitution of the rock formations and their unequal 

 weathering qualities and not to any other cause. The long naiTow 

 neck of land extending between the Cumberland and the Harpeth 

 Rivers in the southern part of Davidson County, does not show any- 

 thing which might be attributed to the action of water in such a body 

 as to terrace the sides of the hills forminc; the shores. The fringed 

 appearance of the escarpment appeal's to indicate a slower and steadier 

 but at the same time a no less equally jjowerful eroding agent as any 

 power that waves might bring to bear upon it. 



If by any means the Central Basin of Tennessee were formed into 

 a lake it would occupy a space 110 square miles less th;m Lake 

 Ontario, but would be about 40 feet deeper. 



Geological Structure of the Highland Rim. 



The geological structure of the Highland Rim ]>re.sents us wiih an 

 •evidence that the position now occupied by the basin was at the close 

 of the Lower Silurian Age, or during the period between the deposi- 

 tion of the last of the Nashville beds and the beginning of the 

 Niagara, or the deposition of the Medina or lowest member of the 

 Niagara group, elevated to a considerable height above the waters of 

 the surrounding sea. The sections shown by the different sides of the 

 basin enables us to ti'ace with considerable accuracy the extent of 

 territory laid dry about this time. A general section of the escarp- 

 ment upon the eastern side shows the total absence of the beds of the 

 Upper Silurian and Lower Devonian formations and the Black Shale 

 of the Upper Devonian lying in contact with the beds of the Hudson 

 River formation, or Nashville rocks. On the northwest, west, south- 

 west and part of the south sides the Niagara including the Clinton and 



