Division of the subject.—Geology was defined to be the science which investigates the former changes that have taken place in the organic as well as in the inorganic kingdoms of nature. As vicissitudes in the inorganic world are most apparent, and as on them all fluctuations in the animate creation must in a great measure depend, they may claim our first consideration. The great agents of change in the inorganic world may be divided into two principal classes, the aqueous and the igneous. To the aqueous belong Rain, Rivers, Torrents, Springs, Currents, and Tides; to the igneous, Volcanoes, and Earthquakes. Both these classes are instruments of decay as well as of reproduction; but they may also be regarded as antagonist forces. For the aqueous agents are incessantly laboring to reduce the inequalities of the earth's surface to a level; while the igneous are equally active in restoring the unevenness of the external crust, partly by heaping up new matter in certain localities, and partly by depressing one portion, and forcing out another, of the earth's envelope.
It is difficult, in a scientific arrangement, to give an accurate view of the combined effects of so many forces in simultaneous operation; because, when we consider them separately, we cannot easily estimate either the extent of their efficacy, or the kind of results which they produce. We are in danger, therefore, when we attempt to examine the influence exerted singly by each, of overlooking the modifications which they produce on one another; and these are so complicated, that sometimes the igneous and aqueous forces co-operate to produce a joint effect, to which neither of them unaided by the other could give rise,—as when repeated earthquakes unite with running water to widen a valley; or when a thermal spring rises up from a great depth, and conveys the mineral ingredients with which it is impregnated from the interior of the earth to the surface. Sometimes the organic combine with the inorganic causes; as when a reef, composed of shells and corals, protects one line of coast from the destroying power of tides or currents, and turns them against some other point; or when drift timber, floated into a lake, fills a hollow to which the stream would not have had sufficient velocity to convey earthy sediment.
It is necessary, however, to divide our observations on these various causes, and to classify them systematically, endeavoring as much as possible to keep in view that the effects in nature are mixed and not simple, as they may appear in an artificial arrangement.
In treating, in the first place, of the aqueous causes, we may consider them under two divisions; first, those which are connected with the circulation of water from the land to the sea, under which are included all the phenomena of rain, rivers, glaciers, and springs; secondly, those which arise from the movements of water in lakes, seas, and the ocean, wherein are comprised the phenomena of waves, tides, and currents. In turning our attention to the former division, we find that the effects of rivers may be subdivided into, first, those of a destroying and transporting, and, secondly, those of a renovating nature; in the former are included the erosion of rocks and the transportation of matter to lower levels; in the renovating class, the formation of deltas by the influx of sediment, and the shallowing of seas; but these processes are so intimately related to each other, that it will not always be possible to consider them under their separate heads.
Fall of Rain.—It is well known that the capacity of the atmosphere to absorb aqueous vapor, and hold it in suspension, increases with every increment of temperature. This capacity is also found to augment in a higher ratio than the augmentation of the heat. Hence, as was first suggested by the geologist, Dr. Hutton, when two volumes of air, of different temperatures, both saturated with moisture, mingle together, clouds and rain are produced, for a mean degree of heat having resulted from the union of the two moist airs, the excess of vapor previously held in suspension by the warmer of the two is given out, and if it be in sufficient abundance is precipitated in the form of rain.
As the temperature of the atmosphere diminishes gradually from the equator towards the pole, the evaporation of water and the quantity of rain diminish also. According to Humboldt's computation, the average annual depth of rain at the equator is 96 inches, while in lat. 45° it is only 29 inches, and in lat. 60° not more than 17 inches. But there are so many disturbing causes, that the actual discharge, in any given locality, may deviate very widely from this rule. In England, for example, where the average fall at London is 24½ inches, as ascertained at the Greenwich Observatory, there is such irregularity in some districts, that while at Whitehaven, in Cumberland, there fell in 1849, 32 inches, the quantity of rain in Borrowdale, near Keswick (only 15 miles to the westward), was no less than 142 inches![262] In like manner, in India, Colonel Sykes found by observations made in 1847 and 1848, that at places situated between 17° and 18° north lat., on a line drawn across the Western Ghauts in the Deccan, the fall of rain varied from 21 to 219 inches.[263] The annual average in Bengal is probably below 80 inches, yet Dr. G. Hooker witnessed at Churrapoonjee, in the year 1850, a fall of 30 inches in 24 hours, and in the same place during a residence of six months (from June to November) 530 inches! This occurred on the south face of the Khasia (or Garrow) mountains in Eastern Bengal (see [map], Chap. XVIII.), where the depth during the whole of the same year probably exceeded 600 inches. So extraordinary a discharge of water, which, as we shall presently see, is very local, may be thus accounted for. Warm, southerly winds, blowing over the Bay of Bengal, and becoming laden with vapor during their passage, reach the low level delta of the Ganges and Brahmapootra, where the ordinary heat exceeds that of the sea, and where evaporation is constantly going on from countless marshes and the arms of the great rivers. A mingling of two masses of damp air of different temperatures probably causes the fall of 70 or 80 inches of rain, which takes place on the plains. The monsoon having crossed the delta, impinges on the Khasia mountains, which rise abruptly from the plain to a mean elevation of between 4000 and 5000 feet. Here the wind not only encounters the cold air of the mountains, but, what is far more effective as a refrigerating cause, the aerial current is made to flow upwards, and to ascend to a height of several thousand feet above the sea. Both the air and the vapor contained in it, being thus relieved of much atmospheric pressure, expand suddenly, and are cooled by rarefaction. The vapor is condensed, and about 500 inches of rain are thrown down annually, nearly twenty times as much as falls in Great Britain in a year, and almost all of it poured down in six months. The channel of every torrent and river is swollen at this season, and much sandstone horizontally stratified, and other rocks are reduced to sand and gravel by the flooded streams. So great is the superficial waste (or denudation), that what would otherwise be a rich and luxuriantly wooded region, is converted into a wild and barren moorland.
After the current of warm air has been thus drained of a large portion of its moisture, it still continues its northerly course to the opposite flank of the Khasia range, only 20 miles farther north, and here the fall of rain is reduced to 70 inches in the year. The same wind then blows northwards across the valley of the Brahmapootra, and at length arrives so dry and exhausted at the Bhootan Himalaya (lat. 28° N.), that those mountains, up to the height of 5000 feet, are naked and sterile, and all their outer valleys arid and dusty. The aerial current still continuing its northerly course and ascending to a higher region, becomes further cooled, condensation again ensues, and Bhootan, above 5000 feet, is densely clothed with vegetation.[264]
In another part of India, immediately to the westward, similar phenomena are repeated. The same warm and humid winds, copiously charged with aqueous vapor from the Bay of Bengal, hold their course due north for 300 miles across the flat and hot plains of the Ganges, till they encounter the lofty Sikkim mountains. (See map, Chap. XVIII.) On the southern flank of these they discharge such a deluge of rain that the rivers in the rainy season rise twelve feet in as many hours. Numerous landslips, some of them extending three or four thousand feet along the face of the mountains, composed of granite, gneiss, and slate, descend into the beds of streams, and dam them up for a time, causing temporary lakes, which soon burst their barriers. "Day and night," says Dr. Hooker, "we heard the crashing of falling trees, and the sound of boulders thrown violently against each other in the beds of torrents. By such wear and tear rocky fragments swept down from the hills are in part converted into sand and fine mud; and the turbid Ganges, during its annual inundation, derives more of its sediment from this source than from the waste of the fine clay of the alluvial plains below.[265]
On the verge of the tropics a greater quantity of rain falls annually than at the equator. Yet parts even of the tropical latitudes are entirely destitute of rain: Peru, for example, which owes its vegetation solely to rivers and nightly dews. In that country easterly winds prevail, blowing from the Pacific, and these being intercepted by the Andes, and cooled as they rise, are made to part with all their moisture before reaching the low region to the leeward. The desert zone of North Africa, between lat. 15° and 30° N., is another instance of a rainless region. Five or six consecutive years may pass in Upper Egypt, Nubia, and Dongola, or in the Desert of Sahara, without rain.
From the facts above mentioned, the reader will infer that in the course of successive geological periods there will be great variations in the quantity of rain falling in one and the same region. At one time there may be none whatever during the whole year; at another a fall of 100 or 500 inches; and these two last averages may occur on the two opposite flanks of a mountain-chain, not more than 20 miles wide. While, therefore, the valleys in one district are widened and deepened annually, they may remain stationary in another, the superficial soil being protected from waste by a dense covering of vegetation. This diversity depends on many geographical circumstances, but principally on the height of the land above the sea, the direction of the prevailing winds, and the relative position, at the time being, of the plains, hills, and the ocean, conditions all of which are liable in the course of ages to undergo a complete revolution.