238 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
channel down nearer to grade, increasing the height of the fall at 
X. The beheaded stream, A 1 , robbed of most of its water by the 
capture, now meanders aimlessly about in a valley wholly inappro¬ 
priate to its diminished size. The tributaries, C 2 , C 3 , C 4 , etc., continue 
to bring in an amount of sediment appropriate to the transporting 
power of the former large stream, but finding only a shrunken rem¬ 
nant of that stream, wholly unable to remove the sediment, they 
deposit much of it in the form of alluvial fans. It is possible that 
such a fan may spread across the valley and so obstruct the stream 
course as to cause a lake to form, as at C 2 ; this lake finally overflows 
into the rapidly forming gorge at X, thus shifting the divide between 
the two drainage basins to the fan at C 2 , and developing a small north¬ 
east-flowing tributary, which we may call the inverted stream. 
A somewhat later stage is shown in figure 8. The falls have been 
pushed far up the main stream, and are being worn back along the 
tributary B, as well. In the latter case, however, we may imagine 
some harder rock to be encountered, so that the process of fall reces¬ 
sion goes on much more slowly. Hence, while the falls on the main 
