In wet ground iron compounds leached by ground water from the soil often collect in reddish deposits a few feet below the surface, where their downward progress is arrested by some impervious clay.  At the bottom of bogs and shallow lakes iron ores sometimes accumulate to a depth of several feet.

Decaying organic matter plays a large part in these changes.  In its presence the insoluble iron oxides which give color to most red and yellow rocks are decomposed, leaving the rocks of a gray or bluish color, and the soluble iron compounds which result are readily leached out,—­effects seen where red or yellow clays have been bleached about some decaying tree root.

The iron thus dissolved is laid down as limonite when oxidized, as about a chalybeate spring; but out of contact with the air and in the presence of carbon dioxide supplied by decaying vegetation, as in a peat bog, it may be deposited as iron carbonate, or siderite.

Total amount of underground waters.  In order to realize the vast work in solution and cementation which underground waters are now doing and have done in all geological ages, we must gain some conception of their amount.  At a certain depth, estimated at about six miles, the weight of the crust becomes greater than the rocks can bear, and all cavities and pores in them must be completely closed by the enormous pressure which they sustain.  Below a depth of even three or four miles it is believed that ground water cannot circulate.  Estimating the average pore spaces of the different rocks of the earth’s crust above this depth, and the average per cents of their pore spaces occupied by water, it has been recently computed that the total amount of ground water is equal to a sheet of water one hundred feet deep, covering the entire surface of the earth.

CHAPTER III

RIVERS AND VALLEYS

The run-off.  We have traced the history of that portion of the rainfall which soaks into the ground; let us now return to that part which washes along the surface and is known as the run-off.  Fed by rains and melting snows, the run-off gathers into courses, perhaps but faintly marked at first, which join more definite and deeply cut channels, as twigs their stems.  In a humid climate the larger ravines through which the run-off flows soon descend below the ground-water surface.  Here springs discharge along the sides of the little valleys and permanent streams begin.  The water supplied by the run-off here joins that part of the rainfall which had soaked into the soil, and both now proceed together by way of the stream to the sea.

River floods.  Streams vary greatly in volume during the year.  At stages of flood they fill their immediate banks, or overrun them and inundate any low lands adjacent to the channel; at stages of low water they diminish to but a fraction of their volume when at flood.