Now without at present going any farther into the causes of volcanoes we can see at a glance that the eruption of the igneous rocks must have created a marked and essential difference in the physical geography of the globe. It is to these eruptions that the dislocation and disturbance of the stratified formations are due. The igneous rocks present ridges in the mountains; sometimes they are rounded at the summits, while the aqueous and metamorphic rocks are disposed in layers.
Fig. 689.—Crater of Popocatapetl.
These two classes in their varieties form the land and the crust of the earth, which is ever being acted upon by air and water. The ice, again, polishes and scratches the valleys in which it moves. The loosened boulders that tumble from the mountains are carried down by the ice, and deposited in the glacier moraine, whence flows a stream. By degrees the stone is ground up, and carried away in the water to form sediment in a “delta” at the embouchure, or to lie beneath the surface and form rock once more. The igneous rocks, composed of lavas and ashes, are volcanic rocks, deposited deep down, and then after the lapse of ages disclosed by the action of air and water.
The consideration of the land and water upon the globe shows us that they are distributed over the earth very unequally. There is nearly three times as much water in our planet as there is land, and these proportions could not be altered without giving rise to phenomena, the results of which cannot be properly estimated. Our earth has an area of 197,000,000 of square miles; about 52,000,000 of this is land, and about 145,000,000 of it water; so about three-quarters of the globe is made up of water. The first portion of our subject therefore should be directed to the examination and consideration of water, and the phenomena which arise from its presence upon the earth.
Fig. 690.—Distribution of land and water.
We need not go into details which every geography indicates. We will try to trace the sources, not the plain effects, which all can afterwards study from special books. In a preceding portion of this volume we have explained the chemical composition of water, and we showed by experiment that it is a fluid composed of oxygen and hydrogen gases, in the proportions of one to two volumes respectively. No matter in what form water may appear,—as water, as ice, or as steam,—these proportions never vary in pure water (see[ p. 352]). But water on the earth is seldom, or never, pure. We know the difficulty we have to procure good drinking water, and though it may be filtered, there will remain natural salts, which are found in different degrees in all water upon the globe. We know the rain, which is perfectly pure when condensed from the clouds, absorbs carbonic acid, etc., from the atmosphere. We have shown how this water as soon as it comes upon the earth attacks the rocks, and as it progresses carries away lime. After descending deep down, it rises again in the form of Springs.
Now what are these springs? They are the result of percolation of rain-water through certain strata. When water falls it is absorbed into the ground, unless it happens to rest upon an impermeable rock, in which case it becomes a rivulet. But it can penetrate between the atoms of many rocks, and thus falls through sand and harder rocks, till it reaches a stratum which will not receive it—like clay. We then find that it will flow away in a spring, or if tapped will be an Artesian well. These water-wells are of very ancient date, but the name is more modern.[31] The springs flow out, and develop, with the assistance of tributaries, into rivers. These again receive more tributaries, which swell the volume of their waters, and widen out, carrying millions of gallons hourly to the sea with sediment and gravel and stone.
