58 J.D. Dana on Denuduation in the Pacific. 
d. But if the slopes are gradual, the rills flow into one another 
from a broad area, and enlarge a central trunk, which continues 
on towards the sea, with frequent additions from either side. The 
excavation above, for a while, is small; for the greater abundance 
of water below, during the rainy seasons, causes the denudation 
to be greatest there, and in this part the gorge or valley most 
rapidly forms. In its progress, it enlarges from below upward, 
though also increasing above; at the same time, the many tribu- 
taries are making lateral branches. 
e. Towards the foot of the mountain, the excavating power 
ceases whenever the stream has no longer in this part a rapid de- 
scent,—that is whenever the slope is not above one or two feet to 
the mile. The stream then consists of two parts, the torrent of the 
mountains and the slower waters below, and the latter is gradu- 
ally lengthening at the expense of the former. 
: the lower waters have nearly ceased excavation, a new 
commences in this part,—that of widening the valley. © 
ess 
The stream which here effects little change at low water, is 
flooded in certain seasons, and the abundant waters act duterully 
against the enclosing rocks. Gradually, through this undermin- 
ing and deuuding operation, the narrow bed becomes a flat strip 
of land, between lofty precipices, through which, in the rainy 
season, the streamlet flows in a winding course. The streamlet, 
as the flat bottom of the valley is made, deposits detritus on 
its banks, which in some places so accumulates as to prevent an 
overflow of the banks by any ordinary freshet. Such is the ori- 
gin 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, 1s 
gradually becoming more and more steep. The rock-material 
upon, consists of layers of unequal hardness, varying 
but little from horizontality and dipping towards the sea, and this 
occasions the formation of cascades, henever 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 pro- 
cess begun, it goes on with accumulating power. The descend- 
ing waters in this spot add their whole weight, as well asa 
greatly increased velocity, to their ordinary force, and the exca- 
vation 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 dis- 
tance 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 exca- 
vation below is concerned ;—though it may continue to inerease 
from the gradual wear and removal of the rocks over which it 
descends : 
ae 
