386 PACIFIC ISLANDS. 
a. Suppose a mountain, sloping like one of the volcanic domes 
of the Pacific. The excavating power at work proceeds from the 
rains or condensed vapour, and depends upon the amount of water 
and rapidity of slope. 
b. The transporting force of flowing water* increases as the sixth 
power of the velocity,—double the velocity giving sixty-four times 
the transporting power. ‘The rate will be much greater than this 
on a descending slope, where the waters add their own gravity to the 
direct action of a progressive movement. 
c. Hence, if the slopes are steep, the water gathering into rills, ex- 
cavates so rapidly that every growing streamlet ploughs out a gorge 
or furrow; and consequently the number of separate gorges is very 
large, and their sizes comparatively small, though of great depth. 
d. But if the slopes are gradual, the rills flow into one another 
from a broad area, and enlarge a central trunk, which with incessant 
additions from either side, descends towards the sea. ‘The excava- 
tion above, for a while, is small: 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, 
while at the same time the many tributaries are making lateral 
branches. 
e. ‘Towards the foot of the mountain, the excavating power gradu- 
ally ceases when the stream has no longer in this part a rapid descent, 
—that is, whenever the slope is not above a few feet to the mile. The 
stream then consists of tivo parts, the torrent of the mountains and the 
slower waters below, and the latter is gradually lengthening at the ex- 
pense of the former. 
fj. After the lower waters have nearly ceased excavation, a new 
process commences in this part,—that of widening the valley. The 
stream which here effects little change at low water, is flooded in 
* It has been shown by Mr. W. Hopkins that the moving force of running water, (this 
force being estimated by the volume or weight of a mass of any given form which it is 
just capable of moving,) varies as the s¢zih power of the velocity. He says “if a stream 
of ten miles an hour would just move a block of five tons weight, a current of fifteen 
miles an hour would move a block of similar form upwards of fifty-five tons; a current 
of twenty miles an hour would, according to the same law, move a block of three hun- 
dred and twenty tons: again, according to the same law, a current of two miles an hour 
would move a pebble of similar form of only a few ounces in weight.” —-Ox the Transport 
of Erratic Blocks, Trans, Camb. Phil. Soc. viii. 1844, p. 221, 233. 
