1871.] 49% [ Lesley. 
The map below will give a better idea than any verbal description of the 
difficult nature of the ground for railroading purposes down Clinch Val-. 
ley. The hills are from 200 to 300 feet high and present bold and massive 
cliffs of Lower Silurian limestone to the river. 
It will also illustrate the general law that the principal rivers and large 
streams of this region of Virginia run in the lower members of the Lower 
Silurian limestone system, as they habitually do elsewhere, near the edge 
of the Freestone Carboniferous land. The cause of this is evident. The 
surface of the country as it is at present has been produced by the 
remoyal of all the geological formations above that which now forms the 
surface. When the Downthrows were first formed the drainage of the 
SKETCH MAP or CLINCH RIVER, 
where tt strikes the al Measures and rebounds, 
at Lick Rus. 
SSS ES u 
= Ay 
AY 
ASS 
NN 
SS 
country was down the face of the Coal Measures to the crack and then 
along the crack sideways. This produced a very slow erosion of the 
Coal Measures, because of their numerous massive bed of sandstone. 
But the drainage along the cracks produced great erosion into the face of 
the sandy and magnesian limestone exposed by it. The drainage from 
above also produced caverns in the limestone. Out of these caverns 
issued streams which swelled the rivers which ran along the cracks. The 
formations over the limestone were worn rapidly away. The face of the 
limestone wall of the crack was worn back. And so, by the time the 
present surface level was reached, the rivers, which originally flowed on 
the Coal Measure side of the cracks, had got their valley-beds fairly 
established on the limestone side of the cracks; and, sometimes, at a 
considerable distance on that side. 
As the lower limestones are massive and very soluble all the streams 
of the region which flow through them have extremely rough and tortu- 
A. P. 8.—VOL. XII.—3K 
