Even larger bergs are sometimes seen to separate from the ice cliff, in this case an instant before or simultaneously, with a loud report, but such bergs float away with comparatively little commotion in the water. [Illustration: FIG. 320.—A northern iceberg surrounded by sea ice.] The icebergs of the south polar region are usually built upon a far grander scale than those of the Arctic regions, and are, further, both distinctly tabular in form and bounded by rectangular outlines (Fig. 321). Whereas the large bergs of Greenlandic origin are of ice and blue in color, the tabular bergs of Antarctica might better be described as _snowbergs_, since they are of a blinding whiteness and their visible portions are either compacted snow or alternating thick layers of compact snow and thin ribbons of blue ice, the latter thicker and more abundant toward the base. All such bergs have been derived from the shelf ice and not from the inland ice itself. Blue icebergs which have been derived from the inland ice have been described from the one Antarctic land that has been explored in which that ice descends directly to the sea. [Illustration: FIG. 321.—Tabular Antarctic iceberg separating from the shelf ice (after Shackleton).] In both the northern and southern hemispheres those bergs which have floated into lower latitudes have suffered profound transformations. Their exposed surfaces have been melted in the sun, washed by the rain, and battered by the waves, so that they lose their relatively simple forms but acquire rounded surfaces in place of the early angular ones (Fig. 318, p. 291). Sir John Murray, who had such extended opportunities of studying the southern icebergs from the deck of the _Challenger_, has thus described their beauties: “Waves dash, against the vertical faces of the floating ice island as against a rocky shore, so that at the sea level they are first cut into ledges and gullies, and then into caves and caverns of the most heavenly blue, from out of which there comes the resounding roar of the ocean, and into which the snow-white and other petrels may be seen to wing their way through guards of soldier-like penguins stationed at the entrances. As these ice islands are slowly drifted by wind and current to the north, they tilt, turn and sometimes capsize, and then submerged prongs and spits are thrown high into the air, producing irregular pinnacled bergs higher, possibly, than the original table-shaped mass.” READING REFERENCES FOR CHAPTERS XX AND XXI General:— I. C. RUSSELL. Glaciers of North America. Ginn, Boston, 1897, pp. 210, pls. 22. CHAMBERLIN and SALISBURY. Geology, vol. 1, pp. 232-308. H. HESS. Die Gletscher, Braunschweig, 1904, pp. 426 (illustrated). WILLIAM H. HOBBS. Characteristics of Existing Glaciers. Macmillan, 1911, pp. 301, pls. 34. Special districts of mountain glaciers:—