which, as they thicken, again unite, and are again broken up into larger masses. These masses, by rubbing against each other, have their edges slightly rounded up, and in this form receive the name of pancake ice.
When a quantity of ice covers the ocean in a wide level sheet of considerable extent, it is called an ice-field. Fields of this kind are often seen by navigators hundreds of miles in extent, and nearly thirty feet thick. Ice of such thickness, however, only shows five or six feet above water. When fields are broken by heavy ocean-swells, the edges are violently forced up, and fall in debris on the surface; thus hummocks or mounds are formed.
When field-ice breaks up under the influence of an ocean-swell, caused by a storm, the results are terrific.
An exceedingly graphic account of an incident of this kind is given by Dr. Brown, in his "History of the Propagation of Christianity." He writes:—
"The missionaries met a sledge with Esquimaux, turning in from the sea, who threw out some hints that it might be as well for them to return. After some time, their own Esquimaux hinted that there was a ground-swell under the ice. It was then scarcely perceptible, except on lying down and applying the ear close to the ice, when a hollow, disagreeable, grating sound was heard ascending from the abyss. As the motion of the sea under the ice had grown more perceptible, they became alarmed,
