74 DR. E. HULL ON THE PHYSICAL GEOLOGY OF 



2. Epoch of Greatest Terrestrial Movements. — The prolonged period 

 of subsidence and deposition above described at length gave place 

 to an epoch of elevation and contraction of the crust, acting with 

 greatest effect and intensity along the line of the Alieghanies, and 

 parallel with the Atlantic sea-board, where the Palaeozoic strata are 

 folded, flexured, and even reversed, along parallel axes, as so 

 admirably illustrated by the late Professor H. P. llogers *. The 

 foldings of the strata, it is well known, generally subside in a 

 westerly direction towards the Valley of the Ohio, and ultimately 

 pass into widely extended dome-shaped centres of elevation with 

 intervening areas of depression. Amongst the former are the 

 " Cincinoati uplift " and the anticline of the Nashville Silurians; 

 amongst the latter is the region of the Cumberland Plateau, which 

 lies aloug the centre of a broad syncline. 



'6. Direction of Greatest Vertical Movement and Erosion. — From 

 what has been said, it clearly follows that the greatest amount of 

 vertical movement, consequent on powerful lateral thrust, was along 

 the Archaean Protaxis of the Alleghanies. All along this line the 

 Palaeozoic strata were elevated thousands of feet above the ocean, 

 and subjected in consequence to great denudation ; this process was 

 doubtless facilitated hj the flexures and fissures accompanying the 

 movement. Away from this axis of disturbance, the strata (as 

 has been already observed) were but slightly moved, with the result 

 that they remained under water and undenuded, or but slightly 

 emergent, long after those on the border of the Archaean Protaxis 

 were being subjected to extensive erosion. 



Under these conditions denudation proceeded more rapidly along 

 the tract bordering the Protaxis, and especially along the arches or 

 anticlinal flexures. The synclines, or trough-shaped areas, were 

 protected from erosion to a greater or less degree. In the region 

 with which we are specially concerned, the line of theUnaka Pange 

 and Blue Mountains, which was perhaps never altogether submerged, 

 was upraised gradually into high land. The Cambrian and ISilurian 

 strata were subjected to erosion ; and streams carrying the materials 

 flowed down the flanks of the emergent land into the sea or estuary 

 to the westward. This process was going on all through the Meso- 

 zoic period. As time went on, these western tracts, wherever in 

 the line of anticlines, were themselves elevated and eroded, and 

 ultimately the synclines themselves ; but the necessary result of 

 this unequal process of erosion would be to leave the synclinal tracts 

 relatively higher than the anticlines. At a later epoch the Cumber- 

 land Plateau began to be formed by the cutting back of the strata 

 in the direction of their dip ; the massive Carboniferous grits, resting 

 on softer strata largely formed of shales, presenting the necessary 

 conditions for the development of a crested ridge. 



4. Formation of the Cumberland Plateau. — We are now in a 

 position to understand the primary conditions under which this 

 plateau was developed. First there are the required stratigraphical 



* ' Geology of Pennsylrania.' 



