geine, and very often ammonia escapes from turf when treated with caustic alkali."

Professor Schübler says that pure earths, such as sand, lime, magnesia, and gypsum, when dry, are nonconductors of electricity, and that clays are also imperfect conductors. When oblong pieces of all the earths are scraped with a knife, they develop negative electricity. Experiments show that when solutions of humus are exposed to a current of galvanic electricity decomposition immediately ensues.; the geine collects about the positive pole, and the earths around the negative. Dr. Dana supposed that by means of this galvanic agency the rootlets of plants were enabled to attract alkali even from the particles of felspar and mica in the soil, while it was yet in an insoluble condition. If this is true, it would depend, says Dr. Hitchcock, to a considerable extent, on the subdivision of its particles. If this be allowed, the presence of alkali in a soluble condition in the soil is not important when the rocks are present which contain it in an insoluble state, as the rootlets will supply themselves from this source. This may be another of the wonderful provisions of Providence for the wellbeing of the plant, because of the liability of the soluble alkalies to be washed away by rains, while in their original condition these little rootlets alone contain the key to their treasures.

Earths are composed of different substances, be

54

CHEMICAL DISINTEGRATION.

cause the rocks of which they are the particles are go various. Thus the stratum may have been of sandstone, when the soil produced will of course be sandy; if of limestone, then the calcareous element will prevail; if of slate, then clay. The minerals which are generally found in the rocks are quartz, felspar, mica, amphibolite, pyroxenite, talc, serpentine, and diallage. These, according to Boussingault, are formed of metals which enter into the structure of plants. Thus quartz is almost pure silica, or sand.

Sir Humphrey Davy observed, with regard to the chemical causes of disintegration: "The manner in which rocks are converted into soil may be easily conceived by referring to the instance of soft or porcelain granite. This substance consists of three ingredients-quartz, felspar, and mica. The quartz is almost pure silicious earth in a crystalline form. The felspar and mica are very compounded substances; both contain silica, allumina, and oxide of iron. In the felspar there is usually found lime and potassa; in the mica, lime and magnesia. When a granite rock of this kind has been exposed to the influence of air and water, the lime and potassa contained in its constituent parts are acted upon by water, or carbonic acid; and the oxide of iron, which is generally in its least oxidized state, tends to combine with more oxygen. The consequence is that the felspar decomposes, and also the

MECHANICAL CAUSES.

55

mica; the first the most rapidly. The felspar, which is the cement of the stone, forms a fine clay; the mica, partially decomposed, mixes with it as sand; and the undecomposed quartz appears as gravel or sand of different degrees of fineness. As soon as the smallest layer of earth is formed on the surface of a rock, the seeds of lichens, mosses, and other imperfect vegetables, which are constantly floating in the atmosphere, and which have made it their resting-place, begin to vegetate; their death, decomposition, and decay afford a certain quantity of organizable matter, which mixes with the earthy material of the rocks. In this improved soil those perfect plants are capable of subsisting. These in their turn absorb nourishment from water and the atmosphere, and, after perishing, afford new material to that already provided. The decomposition of the rocks still continues; and at length, by such a slow and gradual process, a soil is formed in which even forest trees can fix their roots, and which is fitted to reward the labors of the cultivator."

Boussingault thus gives the mechanical causes effecting segregation. "Water, by reason of its fluidity, penetrates the masses of rocks that are at all porous, and filters into their fissures. If the temperature now falls, and the water congeals, it separates by its dilatation the molecules of the minerals from one another, destroys their cohesion, produces clefts, and slowly reduces the hardest

rocks, first to fragments, then to powder. During its frozen state, the ice may serve as a cement, and hold together the disintegrated particles; but with a thaw, the slightest force, currents of water, or the effect of weight, suffices to carry the fragments to the bottom of the valley; and the rubbing and motion to which these are exposed in torrents, tend to break them still smaller, and reduce them to sand."

It is well known that water, by an apparent exception to a general law, expands with great force when freezing. Over a large extent of surface the effect may be very considerable, and when boulderstones lying in shallow ponds become partially enveloped in the ice, they must feel the effects of this expansion, and be driven toward the shore; since the force must always act in that direction. As nothing exists to bring back the rock to its original position, the ultimate effect must be to crowd it entirely out of the water; and perhaps to this cause we may impute the fact, that on the margin of some ponds we find a ridge of boulders, while the bottom, to a considerable extent, is free from them.1

"There is, however, one agent of excavation that, still operates to some extent, and this is called icefloods. Their effect is most powerful upon the smaller and more rapid streams. Whoever has not

1 Geological Survey of Massachusetts, by Dr. Hitchcock.

witnessed the breaking up of a river in the spring after a severe winter, when its whole surface has been covered by ice several feet thick, has but a faint idea of the prodigious force exerted at such a time. The ice high up the stream is usually first broken in pieces by the swollen waters. Large masses are thus thrown up edgewise, and forced underneath the unbroken sheet, and the whole bed of the river is blocked up-perhaps, too, where the banks are high and rocky. The water accumulates behind the obstruction until the resistance is overcome, and the huge mass of water and ice urges on its way, crushing and jamming together that which it meets, and thus gaining new strength at every step. Often for miles the stream, prodigiously swollen, is literally crammed with ice, so that the water disappears, and a slowly-moving column of ice is all that is seen. This presses with such force upon the bottom and sides of the river-bed as to cause the earth to tremble, with a sound like heavy thunder, for a distance of miles. Sometimes the body of ice becomes so large, and the friction so great, that the waters are unable to keep it in motion, and it stops, while the river is turned out of its channel, and is compelled to flow in a new bed for weeks and even months."

This cause has a wonderful effect in excavating the beds of rivers. It sometimes tears up great rocks, and pushes them for a considerable distance.