928  PHYSIOGRAPHICAL GEOLOGY. [Boox VIL. 
rock-basins and mount up the farther side, smoothing and striating 
ee. rock as it went, it could, to a certain depth at least, erode 
asins. 
In the general subaerial denudation of a country, innumerable 
minor features are worked out as the structure of the rocks contro!s 
the operations of the eroding agents. Thus, among undisturbed or 
gently inclined strata, a hard bed resting upon others of a softer kind 
is apt to form along its outcrop a line of cliff or escarpment. 
Though a long range of such clitis resembles a coast that has been 
worn by the sea, it may be entirely due to mere atmospheric waste. 
Again, the more resisting portions of a rock may be seen projecting 
as crags or knolls. An igneous mass will stand out as a bold hill 
from amidst the more decomposable strata through which it has 
risen. These features, often so marked on the lower grounds, attain 
their most conspicuous development among the higher and barer 
parts of the mountains, where subaerial disintegration is most rapid. 
The torrents tear out deep gullies from the sides of the declivities. 
Corries or cirques, if not originally scooped out by converging 
streamlets (their mode of formation is a somewhat difficult problem), 
are at least enlarged by this action, and their naked precipices are 
kept bare and steep by the wedging off of successive slices of rock 
along lines of joint. Harder bands of rock project as massive ribs 
down the slopes, shoot up into prominent peaks, or, with the com- 
bined influence of joints and faults, give to the summits the notched 
saw-like outlines they so often present. 
The materials worn from the surface of the higher are spread 
out over the lower grounds. We have already traced how streams at 
once begin to drop their freight of sediment when, by the lessening 
of their declivity, their carrying power is diminished (pp. 367, 382). 
The great plains of the earth’s surface are due to this deposit of 
gravel, sand, and loam. They are thus monuments at once of the 
destructive and reproductive processes which have been in progress 
unceasingly since the first land rose above the sea and the first shower 
of rain fell. Every pebble and particle of the soil of the plains, once 
a portion of the distant mountains, has travelled slowly and fitfully 
downward. Again and again have these materials been shifted, ever 
moving seaward. Jor centuries, perhaps, they have taken their 
share in the fertility of the plains and have ministered to the 
nurture of flower and tree, of the bird of the air, the beast of the 
field, and of man himself. But their destiny is still the great ocean. 
In that bourne alone can they find undisturbed repose, and there, — 
slowly accumulating in massive beds, they will remain until, in the 
course of ages, renewed upheaval shall raise them into future land, ~ 
and thereby enable them once more to pass through a similar cycle 
of change. 
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THE END. 
