In every part of the ocean the mariner has to guard against the perils of hidden shoals and sunken cliffs, but the high northern waters are doubly and trebly dangerous; for here, besides those rocks which are firmly rooted to the ground, there are others which, freely floating about, threaten to crush his vessel to pieces, or to force it along with them in helpless bondage. The Arctic navigators have given various names to these movable shoals, which are the cause of so much delay and danger. They are _icebergs_ when they tower to a considerable height above the waters, and _ice-fields_ when they have a vast horizontal extension. A _floe_ is a detached portion of a field; _pack-ice_, a large area of floes or smaller fragments closely driven together so as to oppose a firm barrier to the progress of a ship; and _drift-ice_, loose ice in motion, but not so firmly packed as to prevent a vessel from making her way through its yielding masses. [Illustration: 22. AMONG HUMMOCKS.] The large ice-fields which the whaler encounters in Baffin’s Bay, or on the seas between Spitzbergen and Greenland, constitute one of the marvels of the deep. There is a solemn grandeur in the slow majestic motion with which they are drifted by the currents to the south; and their enormous masses, as mile after mile comes floating by, impress the spectator with the idea of a boundless extent and an irresistible power. But, vast and mighty as they are, they are unable to withstand the elements combined for their destruction, and their apparently triumphal march leads them only to their ruin. [Illustration: 23. DRIFTING IN THE ICE.] When they first descend from their northern strongholds, the ice of which they are composed is of the average thickness of from ten to fifteen feet, and their surface is sometimes tolerably smooth and even, but in general it is covered with numberless ice-blocks or hummocks piled upon each other in wild confusion to a height of forty or fifty feet, the result of repeated collisions before flakes and floes were soldered into fields. Before the end of June they are covered with snow, sometimes six feet deep, which melting during the summer forms small ponds or lakes upon their surface. [Illustration: 24. FORMS OF ICEBERGS.]