This is a subject on which it is needless to enlarge. Every one knows that the tides have the power of transporting solid matter; though most of us, perhaps, do not fully appreciate the magnitude of their accumulated effects, working as they do with untiring energies upon the coasts of islands and continents all over the world. It is not, however, so generally known that the ocean is traversed in all directions by powerful currents, which, from their regularity, their permanence, and their extent, have been aptly called the rivers of the ocean. We do not mean here to inquire into the causes of these currents, upon which the progress of physical science has thrown considerable light: neither can we hope to describe even the principal currents that prevail over the vast tracts of water which constitute about three-fourths of the entire surface of our globe. We shall content ourselves with tracing the course of one great system, which may serve to give some idea of their general character and enormous power.
This system would seem to have its origin with a stream that flows from the Indian Ocean toward the southwest, and then doubling the Cape of Good Hope, turns northward along the African coast. It is here called the South Atlantic Current. When it encounters the shores of Guinea, it is diverted to the west, and stretches across the Atlantic, traversing forty degrees of longitude until it reaches the projecting promontory of Brazil in South America. In this part of its course it is known as the Equatorial Current, because it follows pretty nearly the line of the Equator: it varies in breadth from two hundred to five hundred miles, and it travels at the mean rate of thirty miles a day, though sometimes its velocity is increased to seventy or eighty. Next, under the name of the Guyana Current, it pursues a northwesterly direction, following the line of the coast; and passing close to the island of Trinidad, becomes diffused, and almost seems to be lost, in the Caribbean Sea. Nevertheless, it again issues with renewed energy from the Gulf of Mexico, and rushing through the Straits of Florida at the rate of four and five miles an hour, it issues once more into the broad waters of the Atlantic. From this out it is called the Gulf Stream, and is well known to all who are concerned in Transatlantic navigation; for it sensibly accelerates the speed of vessels which are bound from America to Europe, and sensibly retards those sailing from Europe to America.
The Gulf Stream, however, does not set out on its Transatlantic voyage directly that it issues from the Straits of Florida. It keeps at first a northeasterly course, following the outline of the American continent, passing by New York and Nova Scotia, and brushing the southern extremity of the great Newfoundland Bank. Then taking leave of the land, it sweeps right across the Atlantic. After a time it seems to divide into two branches, one inclining to the south, and losing itself among the Azores, the other bending toward the north, washing the shores of Ireland, Scotland, Norway, and reaching even to the frozen regions of Spitzbergen. The breadth of the Gulf Stream, when it issues from the Straits of Florida, is about fifty miles, but it afterward increases to three hundred. Its color is a dark indigo blue, which, contrasting sharply with the green waters of the Atlantic, forms a line of junction distinctly visible for some hundreds of miles: afterward, when this boundary line is no longer sensible to the eye, it is easily ascertained by the thermometer; for the temperature of the Gulf Stream is everywhere from eight to ten degrees higher than that of the surrounding ocean.[26]
We leave our readers to infer from this brief description how immense must be the power of transport which belongs to such currents as these. They sweep along the shores of continents, and carry away the accumulated fragments of rock, which had first been rent from the cliffs by the waves of the sea, and then borne out to a little distance by the tides: they pass by the mouths of great rivers, and receiving the spoils of many a fertile and populous country, and the ruins of many an inaccessible mountain ridge, they hurry off to deposit this vast and varied freight in the deep abysses of the ocean. There is one circumstance, however, which we ought not to pass over in silence; for it is of especial importance to the Geologist, and might easily escape the notice of the general reader. It is a well ascertained fact that plants and fruits and other objects from the West Indian Islands are annually washed ashore by the Gulf Stream on the northwestern coasts of Europe. The mast of a man-of-war burnt at Jamaica was after some months found stranded on one of the Western Islands of Scotland;[27] and General Sabine tells us that when he was in Norway, in the year 1823, casks of palm-oil were picked up on the shore near the North Cape, which belonged to a vessel that had been wrecked the previous year at Cape Lopez on the African coast.[28] It seems most probable that these casks of oil must first have crossed the Atlantic from east to west in the Equatorial Current, then described the circuit of the West Indian Islands, and finally coming in with the Gulf Stream, recrossed the Atlantic, performing altogether a journey of more than eight thousand miles. From these facts it is clear that, by the agency of ocean currents, the productions of one country may be carried to another that is far distant. And Geologists do not fail to make use of this important conclusion when they find the animal and vegetable remains of different climates associated together in the same strata of the Earth.
CHAPTER IV.
THEORY OF DENUDATION—CONCLUDED.
Glaciers—Their nature and composition—Their unceasing motion—Powerful agents of denudation—Icebergs—Their number and size—Erratic blocks and loose gravel spread out over mountains, plains, and valleys, at the bottom of the sea—Characteristic marks of moving ice—Evidence of ancient glacial action—Illustrations from the Alps—From the mountains of the Jura—Theory applied to northern Europe—To Scotland, Wales, and Ireland—The fact of denudation established—Summary of the evidence—This fact the first step in geological theory.
The next agent of Denudation to which we invite the attention of our readers, is one of which our own country affords us no example, but which may be seen in full operation amidst the wild and impressive scenery of Switzerland. And we know not how we can better introduce the subject than by the solemn address of a great poet, in whom an ardent love of nature was blended with a deep sense of religion. As he stood in the midst of the snow-clad mountains that shut in the valley of Chamouni, his spirit, “expanded by the genius of the spot,” soared away from the scenes before him to the Great Invisible Author of all that is beautiful and sublime in nature, and he poured forth that well-known hymn of praise and worship in which he thus apostrophizes the massive glaciers of Mont Blanc:—