102 GEOLOGICAL SURVEY 
District. Among the mountains stupendous examples occur of the effects of this cause. There 
vast strata are removed, often leaving but an isolated outlier, like Table-rock, to attest their enor- 
mous thickness. But it is not always easy, in the mountains, to separate the effects of the two 
causes, elevation and denudation; and hence I have chosen to illustrate the subject by examples 
found lower down, where denudation was the sole cause. But the most remarkable instance that I 
have ever met with, in any country, of the preservation of an isolated spot, while the surrounding 
surface was depressed by waste, is found about ten miles South of Chesterfield Court House. The 
whole of that part of the District is, on the surface, a light, poor sand, with beds of sandy loam 
and clay below. Near the Court House may be seen large sheets of rock, varying in thickness 
from a few inches to three feet, composed of sand and oxide of iron—the latter acting as a cement 
to the former. The mass varies between iron ore and ferruginous sandstone. It is very indestructi- 
ble, and is sometimes used for the construction of chimneys. 
This rock is widely distributed over the sandy parts of the District. The locality to which I 
have alluded, occurs on Mountain Prong of 'Thompson’s Creek. On approaching this little stream, 
a pretty regular peak is seen rising from the sandy plain, and towering above the tall pines that 
surround it, which suggests, at first, the cone of a volcano, its sides strewed with black scoria.— 
Finding, however, that what appeared like scoria is nothing but the ferruginous sandstone just 
described, and by which the cone is completely shingled from top to bottom, one is next reminded 
of an Indian mound, so regular and artificial does the whole appear. Some of the stones have 
fallen down from the S. E. side, and a ravine has been washed out, where the structure of this 
curious little hill may be studied. It is composed, like the rest of this part of the District, of alter- 
nating beds of gravel, sand, common and porcelain clay, beautifully stratified. On the top isa 
large tablet of sandstone, twelve feet square, which marks the original level of the surface of the 
surrounding country, but which is now elevated 150 feet above it, for that is the height of the hill. 
It is not very difficult to understand how the sandstone, which once covered the surface of a par- 
ticular spot with a horizontal sheet, would fall down as it was undermined by the encroachment of 
denuding forces, and thus completely protect it from further waste. And when no longer under- 
mined the last flag would retain its original horizontal position. Art could devise no more complete 
mode of protecting a hill from all washing than this, for besides the perfect and indestructible cov- 
ering on the sides, the top is protected by a well laid and ample coping stone; and but for the 
ravine, which seems to be of recent origin, it would be as enduring as the pyramids. 
We have here, then, a level-mark, from which we learn that this region has been wasted, by 
denudation, to the depth of 150 feet; for 
the regularly stratified and slightly inclined 
beds of sand and clay, forbid the supposition 
that the surface could have been originally 
very uneven. These beds belong to the 
HKocene formation, and hence this amount 
of degradation has taken place since that 
period. Fig. 23 represents this hill. 
The other instances that I gave were the 
result of an infinitely longer period of time, 
