SECTION I.—ATMOSPHERIC CAUSES.
The oxygen of the atmosphere is capable of uniting with some of the constituents of rocks, by which their cohesion is weakened or destroyed. This is the cause of the rapid disintegration of some varieties of granite. The protoxide of iron which they contain is converted, by contact with the atmosphere, into the peroxide. Its volume is thus increased, and portions of the rock are separated from the mass. When granite or limestone contains sulphuret of iron, the oxygen of the atmosphere, in connection with moisture, combines with the sulphur, forming sulphuric acid, by which limestone and the felspar of granite are rapidly decomposed. Hence, a rock which contains an oxide or sulphuret of iron should not be used for architectural purposes.
Carbonic acid is another constituent of the atmosphere which operates as a decomposing agent. The water that falls from the atmosphere is charged with it, and thus becomes capable of dissolving calcareous rocks. Carbonic acid is thus indirectly the means of the rapid destruction of rocks of this class. It is also believed that carbonic acid enters into direct combination with some of the constituents of rocks, and particularly felspar; for it is found that in those countries where carbonic acid issues in great quantities from the earth, the rocks, especially those which contain felspar, disintegrate rapidly. Masses of many tons’ weight, which appear to be solid granite, after being broken are found to be in such a state of decay that fragments may be reduced to sand between the fingers.
The moisture of the atmosphere has some effect as a decomposing agent. Rocks which are exposed to frequent alternations of moisture and dryness soon crumble into fragments. Rain, falling upon the surface of rock, produces, mechanically, a destroying effect, which is not to be overlooked.
Variations of temperature, especially those alternations above and below the freezing point, have greater influence than any other cause in the destruction of rocks. When the water with which a rock is saturated congeals, the resulting expansion tends to enlarge the interstices, and thus to separate the particles of the rock. When the ice melts, the particles fail to resume the closeness of arrangement with which they were before packed. By frequent repetition of this action, the superficial portion loses its cohesion, and disintegrates. It is also found that in the region of perpetual snow the surface of the mountain masses is covered with rock in a disintegrated or fragmentary state, in greater abundance than below the snow line; but no explanation of this fact has yet been found.
In mountainous regions, electrical discharges and violent storms have some destroying effect. Winds have considerable power in changing the place of earthy matter in a disintegrated state. In deserts, the sands are carried in great quantities to great distances.
The causes now enumerated, when considered separately, and as acting for only limited periods of time, seem hardly worthy of notice; but when considered as operating conjointly, and for indefinite periods of time, they must have produced important changes on the surface of the earth.
From these causes, the surface and ornaments of castles and other ancient edifices, and of boulders, and all insulated rocks, are found to be decayed, and often to a considerable depth. It is from these causes that a soil is produced on every surface of rock which is not so exposed to the action of currents that the debris is removed as fast as it is formed. Hence it is, also, that a slope of detritus is formed at the base of every declivity, so that the ledge appears only at the highest points.
It is from a combination of these atmospheric causes that a large part of the sediment is furnished which brooks and rivers carry away. And when cohesion is not entirely overcome, it is so far weakened that other causes are much more effectual than they would otherwise be, in effecting the disintegration of rocks.