So little is known of the bed of the ocean that no inference can be drawn with regard to its heights and hollows, and what relation its mean depth bears to the mean height of the land. From its small influence on the gravitating force, La Place assumed it to be about four miles. As the mean height of the continents is about 1000 feet, and their extent only about a fourth of that of the sea, they might be easily submerged, were it not that, in consequence of the sea being only one-fifth of the mean density of the earth, and the earth itself increasing in density towards its centre, La Place has proved that the stability of the equilibrium of the ocean can never be subverted by any physical cause: a general inundation from the mere instability of the ocean is therefore impossible. CHAPTER XIII. The Continent of Australia—Tasmania, or Van Diemen’s Land—Islands— Continental Islands—Pelasgic Islands—New Zealand—New Guinea—Borneo— Atolls—Encircling Reefs—Coral Reefs—Barrier Reefs—Volcanic Islands— Areas of Subsidence and Elevation in the Bed of the Pacific—Active Volcanos—Earthquakes—Secular Changes in the Level of the Land. THE continent of New Holland, situate in the Eastern Pacific Ocean, is so destitute of large navigable rivers that probably no very high land exists in its interior, which, as far as it has been explored, seems to be singularly flat and low, but it is still so little known that no idea can be formed of its mean elevation. It is 2400 miles from east to west, and 1700 from north to south, and is divided into two unequal parts by the Tropic of Capricorn; consequently it has both a temperate and a tropical climate. New Guinea, separated from New Holland by Torres Straits, and traversed by the same chain of mountains with New Holland and Van Diemen’s Land, is so perfectly similar in structure, that it forms but a detached member of the adjacent continent.