they passed, and the degree of heat they had acquired, so must the extent of their influence have been at any former period.

59. Rivers are the most important aqueous agents employed in modifying the surface of the globe. Springs, as they issue into open day, naturally seek a lower level; and numbers of them meeting in one channel, form streams, which again join in some still lower valley, where their union produces rivers of various sizes. Rivers may be said to be a species of natural drainage, by which the superabundant moisture which falls on the land is again returned to the sea. They are of all dimensions; in breadth from a few feet to several miles, so shallow that a boy might wade them, or so deep as to float the largest ships, and ranging in length of course from fifty or sixty miles to as many hundreds.

60. The geological action of rivers is twofold; first, by wearing down the land through which they pass, and then by carrying down the material to lakes and seas. Both their degrading and transporting force depends upon their velocity. For example, it has been calculated that a force of 3 inches per second will tear up fine clay, 6 inches will lift fine sand, 8 inches sand as coarse as linseed, and 12 inches fine gravel; while it requires a velocity of 24 inches per second to roll along rounded pebbles an inch in diameter, and 36 inches per second to sweep angular stones of the size of a hen's egg. Rivers, during floods, often acquire a much greater velocity than this, and stones of considerable weight are then borne down by their currents. The degrading power of runningwater depends also upon the kind of material through which it flows; loose soil, clay, and sandstone being easily worn down, while granite or basalt will suffer little loss for centuries. The mere flowing of pure water would exert little influence on hard rocks; but all rivers carry down sand and gravel; and these, by rubbing and striking against the sides and bottoms of the channel, assist in scooping out those channels which everywhere present themselves. The Nerbuddah, a river of India, has scooped out a channel in basaltic rock 100 feet deep. Messrs Sedgwick and Murchison give an account of gorges scooped out in beds of the rock called conglomerate, in the valleys of the Eastern Alps, 600 or 700 feet deep. A stream of lava, which was vomited from Ætna in 1603, happened to flow across the channel of the river Simeto. Since that time the stream has cut a passage through the compact rock to the depth of between 40 and 50 feet, and to the breadth of between 50 and several hundred feet. The cataract of Niagara, in North America, has receded nearly 50 yards during the last forty years. Below the Falls, the

river flows in a channel upwards of 150 feet deep and 160 yards wide, for a distance of seven miles; and this channel has evidently been produced by the action of the river. Such effects as the above are produced by the general or ordinary action of water; but when rivers are swollen by heavy rains, by the sudden melting of snow, and the like, then they act with extraordinary violence. In these cases they overflow their banks, rush with a velocity of 20 or 30 feet per second, tear up the soil, and sweep before them trees, animals, houses, and bridges. The water of all rivers which exert a degrading influence is more or less turbid, and an idea of their power may be formed by observing this fact.

61. The matter which rivers carry down is either deposited along their banks, in lakes, or in the ocean. If they flow sluggishly along a flat valley, the mud and sand which their waters contain gradually falls to the bottom, and there rests as sediment. This sedimentary matter forms what is called alluvial land, and most of the flat and fertile valleys in the world have been so produced. Again, when a lake occurs in the course of rivers, the sediment is there collected, and the water issues from the lake as if it had been filtered. In progress of time, lakes are filled or silted up with this sediment, and their basins appear first as marshes, and latterly as alluvial land. But whatever quantity of matter may be deposited in valleys or lakes, the greatest amount will always be carried down to the ocean, and deposited at the mouth of the river or along the shores. The heaviest material, such as gravel, will fall down first, then the lighter sand, and ultimately the finest mud. The mud of the Ganges discolours the Bay of Bengal to a distance of 60 miles from its mouth; and according to Captain Sabine, the muddy waters of the Amazon may be distinguished 300 miles from the shore.

62. The consequence of this continual seawardcarriage of sedimentary matter is, that at the mouths of most rivers there are alluvial formations, known by the name of deltas; such as those of the Nile, the Ganges, the Niger, &c. They take their name from their resemblance in shape to the Greek letter ▲ (delta); and fre

quently extend over vast surfaces-that of the Ganges being about 200 miles in one direction by 220 in another. They consist of alternate layers of sand, gravel, or mud, according to the kind of material the river carries down. The foregoing cut represents the Delta of the Nile, which is generally regarded as the type of all similar deposits.

63. The geological results effected by the agency of running water are ceaseless and universal. Rivers are gradually wearing down the hills and higher lands, and as gradually silting up lakes and low tracts of valley land. They lay down beds` of gravel, sand, or mud; and these beds, again, enclose trees, plants, the bones and shells of animals, in greater or less abundance. As rivers now act, so must they have always acted, and to this kind of agency must we ascribe the formation of many of the rocks (with their fossils) which now form the crust of the earth both at great depths and at distances now far removed from the sea. We have no actual knowledge of the rivers of the ancient world; but, judging from the extent of sedimentary rocks, they must have been much more gigantic than most of those now existing.

64. Waves, currents, and tides, are also powerful geological agents. Waves are continually in action; and according to their violence, and the materials composing the sea-coast, so is the amount of change produced. Cliffs of sandstone, chalk, clay, or other soft rock, are, year after year, undermined by their force; masses fall down, are soon ground to pieces, and swept off by every tide; new underminings take place, new masses fall down, and thus thousands of acres of land have been reduced to a level with the sea. What the waves batter down, the tides and currents transport to sheltered bays and creeks along the shore; so that, while in one quarter the sea is making encroachments on the land, in another it is accumulating sand and gravel to form new land. The power of waves and currents is much increased by the fact, that rocks are more easily moved in water, and thus gravel beaches are piled up or swept away with apparent facility. The ordinary action of the sea is small, however, compared with what is sometimes accomplished during storms and high inundations; and those who have witnessed the effects of a few successive tides at such periods, will readily form an estimate of what may be accomplished during the lapse of ages.

65. The action of waves, currents, and tides, is varied and complicated; but it may be stated generally, that waves batter down the sea-cliffs, or raise up loose matter from the bottom;

that tidal currents convey the disintegrated matter to more sheltered bays and creeks; and that oceanic currents convey floating material, such as drift-wood, plants, and dead animals, from one part of the ocean to another. Tides rise and ebb from 4 to 40 feet; they enter into certain rivers for many miles; and thus a mingling of fresh water and marine deposits take place. As at present, so in ages past; and by diligently studying the effects produced by waves and tides, the student will be enabled to account for many appearances which the sedimentary rocks present.

EXPLANATORY NOTE.

THE ACTION OF WATER is said to be meteoric when it acts through the atmosphere; fluviatile (fluvius, a river) when it acts by running streams or rivers; lacustrine (lacus, a lake), by pools or lakes; and oceanic, when by the ocean.

SILT.-Mud or sand carried down by any river, and deposited either along its banks or in lakes, is called silt; and when a lake becomes filled with this matter, it is said to be silted up. Silt is generally applied to matter calmly or slowly deposited.

DEBRIS (French)—a term applied to the loose material arising from the disintegration of rocks.

ALLUVIAL (Lat., luere, to wash, and ad, together). Land washed or brought together by the action of water is said to be alluvial.

Most

of the straths and carses in Scotland, and the dales in England, are alluvial; as are also the deltas of all such rivers as the Nile, Ganges, Niger, Mississippi, &c.

SEDIMENT (Lat., sedere, to sit or settle down)-matter settled down from solution in water. If water containing mud be allowed to stand without agitation, the mud will gradually fall to the bottom, and become sediment. Rocks which have been deposited after this manner, such as sandstone, are said to be sedimentary.

DEPOSIT (Lat., de, down, and positus, placed)-applied to matter which has settled down from water. Mud, sand, gravel, &c. are all deposits, and are distinguished by the kind of agency which produced them; such as fluviatile (river) deposits, lacustrine (lake) deposits, marine (sea) deposits, and littoral (sea-shore) deposits.

IGNEOUS AGENCIES.

66. Atmospheric and aqueous agencies may be said to exert themselves similarly in modifying the crust of the globe. Both have a tendency to wear down the dry land; and if this influence went on year after year, without any counteracting force, a time might arrive when hills and plains would be reduced to one uniform level. But the system of nature is beautifully balanced in all its parts, and as one set of agents degrade, another are employed to elevate. Thus the layers of loose material which are at one time spread out in the bottom of

lakes and seas, is at another raised into open day, to form new lands for the support of vegetable and animal existence. The principal agent employed in this elevating process is the Igneous, or that which depends upon some deep-seated source of fire. Hereafter we shall notice the opinions which have been advanced concerning the origin of subterranean fire; at present we have merely to do with its sensible effects.

67. Igneous agency may exert itself either chemically or mechanically; chemically, as in the production of new compounds, gaseous admixtures, &c.; mechanically, as when it elevates and fractures the solid crust of the earth. Its mode of action may be considered under three heads; namely, Volcanoes, Earthquakes, and Gradually Elevating Forces.

68. Volcanoes may be described as vents of subterraneous fire, through which smoke, gaseous vapours, cinders, ashes, stones, and rocky matter in a state of fusion, are discharged. The explosive or expansive force of the internal fire forms a vent for itself in the first instance; this opening is termed the crater, and the matter discharged, gradually collecting around it, produces a mountain of a towering or conical form, like that described by the following figure.

69. Such is the general appearance of isolated volcanoes; but they frequently occur in ranges, producing, by the union of their forces, elevated mountain chains like those of South