the glacier. Sometimes loose stones give rise to irregularities of direction and figure.

33. From glaciers formed in high latitudes near the sea, fragments of various sizes fall into the ocean, and constitute icebergs. The polar seas abound with them at all seasons, and marine currents float many of them into lower latitudes. They are sometimes of great size; one measured thirteen miles in length, and one hundred feet above the water, giving a thickness of seven hundred or eight hundred feet. Floating into warmer water and atmosphere, icebergs melt, depositing their loads of earth and rocks on the bottom of the ocean. Many of them strand upon the Newfoundland Banks, and after heavy rolling and disturbance of the bottom, they melt, and mingle the fragments they conveyed with the sands of the Banks. Similar phenomena, on a smaller scale, are witnessed in lakes and rivers; a fragment of rock is floated to one side and there left by

*The specific gravity of ice is such as to make it float with one-ninth of its bulk above the surface of the water; but icebergs are porous, and float with the largest extremity immersed, so that one-seventh or one-eighth is above the water.

the melting ice; the succeeding season it may be returned by the same agency to the other side.

34. The geological agency of the ocean is, like that of rivers, twofold. 1st, Erosive, wearing away the coast, undermining and excavating cliffs; and, 2nd, transporting and accumulating the detritus to form new land. It produces these effects by means of waves, tides, and currents.

35. The action of waves is incessant, but varies in extent according to the nature of the exposed shore. Since they do not penetrate very deep, and have no progressive motion in the open sea, they do not affect the bottom of the ocean, except where it is very shoal. Cliffs of soft rocks, clay, chalk, or sandstone, are very rapidly undermined and worn away. The hardest rocks, however, can not resist its never ceasing attacks. Headlands of alternately hard and soft rocks, especially if intersected by crevices, are worn away with very great rapidity. The coasts of England exhibit this erosive action of the ocean in a remarkable degree. The cliffs of Yorkshire, it is ascertained by careful measurements, repeated after intervals of many years, are worn away six feet in breadth, annually. The average annual loss on the coast of Norfolk is about three feet. When Mr. Lyell, in A. D. 1829, visited Sherringham, Norfolkshire, he found water twenty feet deep, where forty-eight years before stood a cliff fifty feet high. In the county of Kent, near the mouth of the Thames, stands the Church of Reculver, upon a cliff twenty feet above the sea. In the time of King Henry VIII. the distance between the church and the brink of the cliff was one mile. The following cut represents the appearance of the spot in A. D. 1781, when the encroachments of the sea had attracted notice, though considerable space, with

C

other buildings, intervened between the churchyard and the cliff. The walls of an ancient Roman fortification

which was two hundred and forty feet nearer the sea than the church was, had recently been undermined and precipitated into the ocean. In A. D. 1804, a part of the churchyard with some adjoining houses was washed away, and the ancient church with its two lofty spires, a well known land-mark, was dismantled, and abandoned as a place of worship. Figure 16 represents it as it appeared in A. D. 1834. It would probably long since have fallen, had not the incursions of the ocean been checked by an artificial causeway of stones and large wooden spiles driven into the sands to break the force of the waves. The isle of Sheppey, seen at some distance from the main land, on the right hand, in figure 15, which is six miles long and four broad, is continually undergoing abrasion, having lost

Fig. 16.

There are other

fifty acres within the last twenty years.* instances in which islands in this (German) ocean, and important seaport towns, have been entirely obliterated.

36. The English channel probably owes its origin to the erosive action of the ocean, the geological features of the coasts of England and France, clearly indicating that they were formerly united. During the thirteenth century, a channel half as wide as the English channel, was excavated in the north of Holland, separating Friesland from the main land. Immense labor is continually expended in Holland to prevent incursions of the ocean, which

threaten to inundate much of the country, and destroy the
cities of Amsterdam and Ley 'en.
The effect of this constant
abrasion on exposed coasts, is seen in the production of
caverns, bridges, and isolated pinnacles of rock, as exem-
plified by the chalk "Needles" of the English coast, and
the "Drongs" of the north of Scotland. Figure 17, pre-
sents a view of the cluster of rocks seen to the south of the
Hillswick Ness, one of the Hebrides. These granite rocks
are all that remain of a former island, which may, at an
earlier period, have been a promontory of the main land.

ocean.

Hillswick Ness, Hebrides Islands, Scotland.

37. The coasts of New England and Nova Scotia also exhibit striking instances of the abrading power of the Boston harbor has been formed by this agency; the outermost islands in it, exposed to the violence of the waves, consist of bare rock; and the more sheltered ones are continually losing a portion of their covering. At Cape May, on the north side of Delaware Bay, the sea has