STRUCTURE AND SURFACE FEATURES 417 
of the land, while the subsequent erosion of that surface 
slope, guided and influenced by the varying character and 
structure of the rocks, has determined the lines. followed 
by tributary streams and brooks. 
A typical river shows an upper or torrent-track, a middle 
or valley-track, and a lower or plain-track. In the torrent- 
track, erosion is at a maximum and deposition of sediment at 
a minimum ; in the valley-track, erosion does not proceed so 
rapidly, while here and there considerable deposition may 
take place; in the plain-track, erosion practically ceases and 
deposition is at a maximum. The plain-track may therefore 
be looked upon as the base-level to which every river strives 
to reduce its bed. As erosion proceeds, the plain-track 
gradually extends inland so as to gain upon the valley-track. 
The latter in like manner is continually encroaching upon the 
torrent-track, while the torrent-track in its turn constantly 
eats into the high ground where it takes its rise. In the 
earlier stages of valley-formation, it is obvious that the 
original configuration of the surface and the varying character 
of the rocks and rock-structures may cause the appearance of 
many cascades, waterfalls, and rapids in all parts of a young 
rivers course. But such obstructions tend gradually to dis- 
appear as erosion proceeds, hard rocks and resistant rock- 
structures are eventually reduced, and an equally graded 
channel finally results. The stage of maturity has now been 
reached—the valley showing a true curve of erosion—being 
relatively steep in its upper course, but rapidly flattening out 
as it descends to the base-level. Hence in all regions which 
have been exposed to the action of subaérial erosion for a 
prolonged period of time, considerable waterfalls ought not 
to occur. If they should appear in a long-established hydro- 
graphic system, we may suspect that the drainage-system 
after having attained maturity has subsequently been inter- 
fered with. Waterfalls cannot be of any great age. Sooner 
or later they must be cut back and replaced by ravines or 
sorges. Their presence, therefore, shows either that the 
valleys in which they occur are throughout of recent age, and 
that the rivers have not yet had time to reduce such 
irregularities, or that the drainage-system, if long-established, 
has since been disturbed by some other agent than running 
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