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Pilling's Cascade, Utah. (Showing type of waterfall in massive rocks which are cut by joints. U. S. Geol. Surv.)

of the earth. We must be prepared at the outset to look past this simple fact of rainfall and to conceive the physical history of the drop of water since it left the surface of the earth in its journey through the clouds and back to earth again.

The story of the raindrop before it comes to the earth is very simple. The heat from the sun, aided in a small measure by the heat from all the stars, vaporizes the water from the earth's surface, mainly from the sea, and removes it in the state of vapor to a height of many thousand feet above the earth's surface. It is maintained there by

the heat which it has absorbed, and thus the main spring of the rain is in the sun. After abiding awhile in the upper regions of the atmosphere, by some of the many chances which beset the clouds, the vapor is cooled; it condenses from the loss of heat and falls as rain or snow. The circumstances of our imaginary mountain top, if that summit be at a considerable height above the sea, favor the cooling of the cloud and therefore the precipitation of this rain. These uplands retain the cold of winter, and during night they pour forth their heat by radiation through the thin air, with more rapidity than the lower lands, which are covered beneath a thicker blanket of atmosphere.

When the drop of rain falls to the earth's surface, if it be of ordinary size, it gives a sensible blow. If that surface be covered with a thin layer of scattered sand-grains or small pebbles, we may observe that the bits of rock dance about and thus apply a little of the force which comes from the drop, to

into rivulets; or if it be covered with mosses, or the thin layer of porous soil common to mountain-tops, it may for a moment disappear from sight in the spongy mass; but a little farther down, we find that it is gathered in rivulets, which quickly join together, so that in descending even a hundred feet below the summit, in a time of rain, we find a number of shallow valleys, each occupied by a little rivulet. The union of these streams gives us one of more power, which may be taken as a typical mountain-torrent. We observe that such a stream descends with considerable rapidity; it is rare indeed that it does not have a fall of more than fifty feet to a mile. The rate of fall in steep-faced mountains often amounts to as much as five hundred feet in that distance. As soon as the stream is more than two or three feet wide and a foot in depth, we begin to see evidences of its energy. Even if the fall be but at the rate of fifty feet to the mile, we shall find that such a stream is able to urge for

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(Showing channel embarrassed by masses of stone fallen from the sides of the valley.)

rub the stone on which they lie. At first, the water spreads over the earth's surface as a thin sheet, but as that surface is never perfectly level, it is, provided the rock be bare, quickly gathered

ward with great violence masses of stone several inches in diameter. If we roll a stone the size of a man's head into the channel, it is swept along, bumping violently against the obstacles it encounters,

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ing soil. This mass of broken-up rock is constantly slipping down the sides of the valley. Every time the winter frost seizes it, it expands a little, and is thus shoved downward; frequently, when soaked with water, great sheets of it slip swiftly, as mud-avalanches, into the stream. In this way the torrent is always

after a few miles of course, though the brook steadily gains in volume by the contributions of tributary streams, it gradually diminishes the swiftness of its descent. At a certain point it ceases to bear onward all of the larger stones which come into its possession. These fragments gather upon the banks, form

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Valley Showing the Beginning of New Terraces, just Below the Torrential Portion of the Stream.

provided with fragments which it may grind up into pebbles, sand, and mud, and bear onward to the fields below. In times of drought, these stream-beds are occupied by rivulets of clear water, and at such periods the observer gains no idea of the vigor with which the mill works; but in times of heavy rain he will find the water turbid with sediment made by the attrition of pebbles against the bordering walls of the stream and upon each other. He then sees whence come the sediments which are so important a feature in the lower portions of the river-system. From any commanding elevation in a mountain district, we may see scores or hundreds of those torrent-beds within one field of view. In periods of heavy rain, the roar arising from the moving stones is often a very striking feature.

Descending the channel of any of these mountain torrents, we find that

ing a rude terrace. Still further down, where the slope is less considerable, the smaller pebbles are left behind, crowded into the interstices of the larger fragments. The terrace becomes more distinct, vegetation gathers upon it, and the waste of the plants forms a soil which partially levels off the surface. Further on, we come to the field where the annual overflow of the stream during the spring floods heaps a quantity of the sand and mud upon this foundation of coarser material; we then have the beginning of the alluvial terrace. At first

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