THE CENTRAL BASIN OF TENNESSEE. 79 



It has already been noticed that nowhere along the escarpment or 

 slope of the eastern side of the Basin is an outcrop of the Niagara to 

 be found. Even the deep gorge made by the Caney F.rk on the 

 east does not show it, and it is not to be found in the valley of the 

 Elk Eiver in the southeast. Beds of the Niagara age, however, are 

 found in East Tennessee, and these extend as far west as the 

 Sequachee Valley on the Alabama State line and run in a north- 

 easterly direction to the Kentucky boundary at the Cumberland Ga]x 



These lines of Niagara outcrops pass clear across the State into 

 Alabama on the south and Kentucky upon the north. The nearest out- 

 crop of Niagara beds in Kentucky is at Liberty, Casey County. 

 — {P7x>ctor, Geological Map, 1887). As they do not appear in the 

 gorge of the Cumberland as far north as Port Burnside, we may safely 

 suppose that the two lines of exposure in Tennessee unite somewhere 

 in the district between Port Burnside and Liberty. The southern 

 junction of the two takes place in the State of Alabama within a short 

 distance south of the line between the two States. 



With these two boundary lines it will be an easy matter to trace 

 the extent of territory raised above water, or at least elevated suffi- 

 ciently high to prevent the formation of rocks of the Niagara group, 

 by the general disturbance following upon the close of the lower Silu- 

 rian. It will be seen also that the greater part of the area now occu- 

 pied by the Central Basin, being devoid of the Niagara formations, 

 must have been above water at the time these rocks were being laid 

 down. 



Passing west toward the Tennessee River Yalley another member 

 of the Upper Silurian makes its appearance. It is doubtful whether 

 the Lower Helderberg is to be found anywhere ai'ound the escarp- 

 ment margin of the Basin. It is, however, found occupying a nan-ow 

 strip of country along the Tennessee Piver in the western part of the 

 State. It attains its maximum thickness in the river valley, and 

 thins rapidly out on coming east. Lower Helderberg in Tennessee 

 consists of a series of light blue limestones, often shaly, highly fos- 

 siliferous and frequently containing cherty layers, especially on its 

 upper part. It has a maximum thickness of 70 feet, and is altogether 

 confined to the western part of the State. It is not known in Eastern 

 Tennessee. 



