SUBTERRANEAN SPRINGS. 219 river, and when, from long continued rains, or from melting of snows and ice, the brooks and tributary streams are swollen, a flood of water is poured down to the ocean, which bears with it materials transported a thousand miles, and in quantities of which we can form little conception. The water, which falls upon the surface of the earth and penetrates its upper soil, and is thus protected from evaporation, descends lower and lower, until it meets some impervious bed of clay, or marl; here it accumulates and forms a hidden pond, and slowly undermines whole tracts of country, and in the course of ages subterranean rivers are formed. The various mineral. ingredients dissolved in water, are borne up by springs, and again flowing over or through the porous sands, form limestones, sandstones, and ironstones; and thus continually the process is going on. The action of all springs, and running waters, is to level the surface of the earth. The streams, which always flow from an elevated source, bear down the disintegrated portions of mountains and hills, and tend continually to fill up the bed of the sea. Unless a counterbalancing cause existed, and the elevation was made to compensate this continual degradation or levelling, the whole dry land would ultimately disappear. We find in earthquakes and other volcanic effects the elevating power; and although, as we shall presently show, the sea may gradually encroach upon the shores of one country, yet the lands of another will be gradually upheaved, and something like a balance will be maintained. Minute therefore as are the transmutations which are going on continually around us, and by which, long since, in the same quiet manner, the leaf that floated down the stream, a thousand years ago, and the insect that dropped into water, have been incrusted, and preserved with a fidelity which mocks the sculptor's art, yet we see that processes like these, have "Turned the ocean-bed to rock, And changed its myriad living swarms To the marble's veined forms." "How marvellous" observes Sir Humphry Davy, "are those laws by which even the humblest types of organic existence are preserved, though born amidst the sources of their destruction; and by which a species of immortality is given to generations, floating, as it were, like evanescent bubbles on a stream raised from the deepest caverns of the earth, and instantly losing what may be called its spirit in the atmosphere." AQUEOUS CAUSES OF CHANGE. 221 CHAPTER V. Currents. Thy shores are empires, changed in all save thee- Byron. WE are, now to consider the remaining aqueous causes of change, currents and tides. The joint action of these produce mutations of great geological interest. The tides, or the great tidal waves which flow over the surface of the ocean at stated intervals, are mainly caused by the attraction of the moon, and hence we may show, and by no very extended chain of causation, that the effect of the moon in altering and keeping in a state of perpetual mutation the face of the earth, is by no means inconsiderable. A more remote cause, the rotation of the earth upon its axis, produces in part at least, great currents which constantly flow in vast circuits in the Atlantic, Pacific, and Indian oceans. In addition to the circular currents which thus flow through the oceans which we have named, there are immense bodies of cold water continually moving from the polar regions towards the central portions of the earth, and, as these currents are exhibited superficially, or on the surface, bearing down immense fields of ice, and since the storms and fogs of those regions are not sufficient to supply this waste of the waters, we may infer that an under current of warmer water passes continually from the equatorial to the polar regions. The polar current of the southern regions seems to be more powerful than that of the northern. Ice islands from 250 to 300 feet above the level of the sea, have been occasionally seen off the Cape of Good Hope, and were therefore of immense bulk, as for every solid foot seen above, there must have been at least eight cubic feet below water. The wood cut below exhibits one of these ice-islands, sketched by Capt. Horsburgh; it was seen off the Cape of Good Hope, in April 1829; it was two miles in circumference and about 150 feet high, appearing like chalk when the sun was obscured, and having the lustre of refined sugar when the sun was shining upon it. Undoubtedly the principal causes of Oceanic currents are the trade-winds, of which we have already spoken. These blowing at first directly from the north and south, over the surface of the water, move the floating ice, and superficial water, in the same general direction, thus at length generating a strong polar current. The south polar current being less intercepted by the peculiar formation of the antarctic lands, than the northern, is perceptible in much higher southern latitudes than the current from the north. A manifest influence is thus exerted upon the climate, to which we shall again allude. The rotation of the earth, when the waters have been set in motion from the north to the south, causes a great change in the general direction of these currents precisely upon the same principle which has long been recognized in the case of trade winds. For example, the current which flows north from the Cape of Good Hope towards the Gulf of Guinea, has a rotary velocity when it doubles the Cape of about 800 miles per hour, but when it reaches the equator, the surface of the earth is there whirled around at GULF STREAM. 223 the rate of 1000 miles an hour, or 200 miles faster. Now if the water was to be suddenly transferred from the Cape to the equator, this deficiency of motion would cause, (inasmuch as the earth rotates from west to east) a very strong current flowing westward at the rate of 200 miles an hour; or with sufficient power to submerge the western continent. No disturbance however occurs, for the water, as it advances into new zones of sea which are moving more rapidly, gradually acquires the different velocity by friction, so that a gentle easterly, or south-easterly current is the result. When the water flows from equatorial to polar regions, a contrary current is produced; thus the Gulf Stream, issues from the Bahama Channel with a rotary velocity of 940 miles an hour, but when it reaches titude 40°, the water is there moving with a rotary velocity of 766 miles an hour, or 174 miles an hour slower, hence a westerly or south-westerly current is the result from the excess of rotary motion retained by the stream. Having shown some of the causes that produce oceanic currents, we will now consider more in detail the most important. From the best accounts which we have been able to obtain, there seems to be a general set of the waters westward from the western coast of Peru. This current flows nearly westward, but is not much perceived until its entrance into the Indian Ocean, when, strengthened by the northerly currents flowing from the North Pacific, it flows along the east coast of of Africa; after passing through the Mozambique Channel, between Madagascar and the continent, it unites with another current from the Indian Ocean, and is deflected by the Lagullas banks, which lie off the southern point of Africa, around the Cape of Good Hope. The collective stream is about one hundred and thirty miles in breadth and from 70 to 80 warmer than the neighboring water, and runs from the rate of two and a half to more than four miles an hour. The Lagullus bank rises from an immense depth to within one hundred fathoms of the surface, and has perhaps been formed by the joint action of a south-eastern and north-eastern current, which meet here. As the main body of the current does not flow over this bank we may conclude its total depth to be much more than one hundred fathoms. We give here a little chart showing the |