Jay. 1911.] BOTANICAL SOCIETY OF EDINBURGH 107 
of what formerly existed. Furthermore, the upper valleys 
are of the characteristic U shape, but are now filled to a 
great depth with shingle, the lofty mountains rising abruptly 
at a steep angle on either side. 
The surface of a river-bed, this being really a narrow 
stony plain, to the eye looks quite flat, as indeed it is in 
cross-section, though the gradient is more or less steep, the 
steepness increasing rapidly as the source of the river is 
neared. Issuing from its ice cave, the river is generally at 
first in one stream, the steep gradient and great size of the 
rocks preventing its wandering, but lower down the bed it 
soon divides, and at times there may be a dozen or more 
shallow but rapid streams, bounded perhaps by rudimentary 
terraces, the number of streams depending altogether upon 
the water-supply. The streams themselves do not occupy 
permanent channels by any means; on the contrary, there 
is a constant but generally slow change in progress, though 
in time of flood movement can be so rapid that high and 
dry shingle occupied by vegetation may be swept away, a 
stream or a naked stony surface subject to more or less 
frequent submersion occupying its place. On the other 
hand, a former stream-channel and the ground adjacent 
may, in their turn, be brought beyond the influence of the 
rapidly-flowing water, for a longer or shorter period, and 
become at once suitable for plant life. 
As already mentioned, in these upper river-valleys per- 
manent high terraces are wanting, those which occur being 
merely temporary and quite low structures, the super- 
abundance of material carried by the streams altogether 
overcoming their erosive power, so that they build but do 
not erode. The river-bed itself is virtually flat, but the 
surface is far from even. Large stones, or even rocks, stand 
up above the general level; there are dry watercourses, 
long since abandoned by their streams, and others only 
temporarily dry. Islands, small or large, remnants of an 
older flood-plain, of irregular form with flat surfaces, bear- 
ing perhaps a scanty plant-covering, surrounded by water 
during the heaviest floods, show here and there upon the 
open river-bed. Close to the slopes, on either side, but 
never forming a continuous margin, there are frequently 
other pieces of the old flood-plain, “river flats” they are 
