historic times, or even before the country was inhabited by man, may be observed in some parts of the south-west coast of Ireland. On the coast west of Bearhaven, in county Cork, and west of Brandon Head, in county Kerry, as also in Derrymore Glen, between the mountains called Baurtregaum and Cahirconrea, there are great cliffs, where the rocks are thrown into positions which appeared puzzling and abnormal, until it was ascertained that they formed parts of gigantic landslips. Masses of land, with cliffs 800 feet high, were then seen to be nothing but a confused heap of broken ruins, their cracks and dislocations being superficial only, and not extending below the level of the

sea.

1 Jukes' "Manual of Geology," second edition, p. 102.

CHAPTER VIII.

FLUVIATILE AGENCIES.

1.-Action of Rivers.

OW a River is formed.-A river results from the

from the union of minor streams or brooks. The feeders of a stream or river are called its tributaries; the area which is drained by the whole number of these tributaries is called the area of drainage, and a river with its tributaries is often spoken of as a system of drainage. The sources of supply are the rain which runs off the ground within the drainage area, and the springs which rise to the surface within that area. Where a river has its sources in mountains that are permanently snow-clad, the melting snow becomes a source of supply in summer. The ridge or line of country from which the rainfall runs off in different directions is called a watershed, or water-parting.

Laws of Flow.-The rate at which water flows from a range of hills to the sea varies greatly in different parts of its journey, because the slope of the watercourse differs in different places. The velocity of a stream at any one place depends primarily on the inclination of the channel at that place, and secondarily on the volume of water passing through it.

The inclination of the channel varies from a steep slope, forming rapids, to a nearly level plain, through which the water is only propelled by the force of the stream behind it. The channel of a mountain torrent often has a slope of over 200 feet per mile; thus the Reuss, between Audermatt and Amsteg, has a fall of about 212 feet per mile. As examples of the channel slopes lower down the valley of a

mountain river we may take the case of the Inn between St. Moritz and Jenbach. Between St. Moritz and Samaden (3 miles) the average slope is 65.3 feet per mile, between Samaden and Landeck (80 miles) it is about 39.3 feet per mile, from Landeck to Innsbruck (53 miles) it averages 13.8 feet, and from Innsbruck to Jenbach (23 miles) about 4 feet per mile.' When a river reaches great plains that are not far above the sea-level its slope is often only a few inches per mile.

Again, the velocity of a current varies with the volume of the streams. If two streams have the same slope for a given distance, but there is more water in the one than in the other, the larger stream will have the greater velocity. Also if a stream is in flood its velocity at a given spot is greater than when there is only the usual volume of water.

Further, not only does the velocity of a river vary at different times and in different parts of its course, but it also varies in different parts of the same cross-section of the current, on account of the friction between the flowing water and the river-bed. Thus the middle of the surface of the stream flows faster than the sides, and the water at the bottom travels slowest, the velocity of the bottom water being often only half that of the surface water in mid-stream.

With regard to the actual velocities of streams, it appears that, except in falls and rapids, the fastest currents do not exceed 20 miles an hour, and a current of 8 or 10 miles an hour at the surface is a rapid'stream, capable of moving large pebbles. It is stated that the average speed of an ordinary river-current varies from 6 miles an hour when the stream is high and swollen, to about 1 mile an hour when the water is low. The Ohio river at Cincinnati, where its fall is 4 inches to the mile, has a mean surface speed of 1 mile an hour when the water is low (i.e., 6 feet deep), when the water is high (54 feet deep) the average current is nearly 6 miles an hour, the actual rate in the middle being 6.35 miles, and at points half way toward either bank 5.85 miles per hour.2

Prof. T. G. Bonney, "Geol. Mag." Dec. 3, vol. v. p. 54. 2 Hinman's" Eclectic Physical Geography," 1889, p. 213.

Transport of Detritus.-Everyone knows that a mountain torrent rolls pebbles along its bed, and that a river in flood is turbid with the mud which it carries; but the geologist requires some more definite information on this important subject. In studying the power of running water to transport rock-material from higher to lower levels there are three primary facts to be remembered :

:

1. Loss of Weight.-All earths and stones lose one-third of their weight when suspended in water, consequently the force required for lifting them is only two-thirds of the force necessary to lift them through air; and still less force is required to push or roll stones along the bed of a

stream.

2. Shape of the Fragments.-Rounded pebbles can be rolled along by a current of water which would be quite unable to move flat slabs of the same weight; conversely, flat slabs or flakes would sink more slowly than rounded fragments of the same cubic contents. Flakes of mica might therefore be transported onwards where grains of quartz would sink, though the latter might be lighter than the former.

3. Velocity of Current.-As already mentioned (p. 126), the velocity of a stream depends upon the volume of water and the declivity of the channel. An increase in either of these factors quickens the current, and increased velocity_means increased power to transport material. Mr. A. Tylor made experiments from which he arrived at the conclusion that the velocity increases as the cube root of the increase of volume on the same slope, and as the cube root of the increase of slope if that is increased and the volume of water is unaltered.' We learn from Mr. W. Hopkins that the transportive power of water increases as the sixth power of the velocity of the current. Thus if the velocity is doubled its motive power is increased 64 times; if its velocity is trebled, its motive power is increased 729 times, and so on."

From Mr. Hopkins' law above quoted we can express the transportive power of streams in terms of the weight

of pebbles moved. Thus if a current of 2 feet per second moves a pebble of half an ounce weight, a current of 4 feet per second will move one of 32 ozs., or 2 lbs. ; a current of 6 feet per second will move one of nearly 23 lbs., and a current of 8 feet per second will move one of 128 lbs. The detritus which is carried along by a stream has for

the most part been carried into it by rain and by landslips, but a smaller quantity is derived from the banks on either side, and from its bed by the erosive action of the stream itself.

The blocks accumulated in a mountain torrent are usually

K