ORIGIN OF VALLEYS. 387 
certain seasons, and the abundant waters act /aterally against the 
inclosing rocks. Gradually, through this undermining and denuding 
operation, the narrow bed becomes a flat strip of land between lofty 
prectpices, through which, in the rainy seasons, the streamlet flows in . 
a winding course. ‘The streamlet, after the flat bottom of the valley 
is made, deposits detritus on its banks, which in some places so accu- 
mulates as to prevent an overflow of the banks by any ordinary 
freshet. Such is the origin of the deep channels with a riband of 
land at bottom that cut through the “dividing plain” of Oahu, and 
which are common towards the shores of many of the Pacific islands. 
g. The torrent part of the stream, as it goes on excavating, is 
gradually becoming more and more steep. ‘The rock-material ope- 
rated upon, consists of layers of unequal hardness, varying but little 
from horizontality and dipping towards the sea, and this occasions the 
formation of cascades. Whenever a softer layer wears more rapidly 
than one above, it causes an abrupt fall in the stream: it may be at 
first but a few feet in height; but the process begun, it goes on with 
accumulating power. ‘The descending waters in this spot add their 
whole weight, as well as a greatly increased velocity, to their ordinary 
force, and the excavation below goes on rapidly, removing even the 
harder layers. ‘The consequences are, a fall of increasing height, and 
a basin-like excavation directly beneath the fall. Often, for a short 
distance below, the stream moves quietly before rushing again on 
its torrent course; and when this result is attained by the action, the 
height of the fall has nearly reached its limit as far as excavation 
below is concerned, though it may continue to increase from the 
gradual wear and removal of the rocks over which it descends. 
h. As the gorge increases in steepness, the excavations above deepen 
rapidly,—the more rapid descent more than compensating, it may be, 
for any difference in the amount of water. Moreover, as the rains are 
generally most frequent at the very summits, the rills in this part are 
kept in almost constant action through the year, while a few miles 
nearer the sea they are often dried up or absorbed among the cavernous 
rocks. The denudation is consequently at all times great about the 
higher parts of the valley, (especially after the slopes have become 
steep by previous degradation,) and finally an abrupt precipice forms 
its head. 
2. The waters descending the ridges either side of the valley or 
gorge, are also removing these barriers between adjacent valleys, and 
are producing asa jirst effect, a thinning of the ridge at summit to a 
