TOPOGRAPHY. This immense labyrinth of isl ands forms that part of a vast submarine plateau which has emerged above the ocean. The sur rounding waters are shallow, for the most part not over 200 feet in depth, ,showing that the wide plateau on which the islands stand nearly approaches the But strewn here and there over the sea floor are troughs and hol lows and wide depressions, particularly to the west of Luzon and Mindanao, where greater depths are found, There is nothing approaching oceanic depths till the eastern edge of the submarine plateau descends to the Pacific deeps from 100 to 300 miles east of the archipelago. In the south three lines of islands stretch like isthmuses between the main archipelago and the southern lands. In the northwest is the most regular and best developed of these isthmuses, stretching from Mindoro to North Borneo, the long, narrow island of Palawan forming more than half its extent. In the centre the Sulu Archipelago connects the western terminus of Mindanao with the northeastern point of Borneo; and in the southwest the great peninsula of Cele bes with the Sanguir Archipelago and other islands forms another isthmus, sweeping around to the south of Mindanao. These ridges are ex
tended throughout the archipelago in the form of mountain ranges, from south to north, and form a large part of its relief. From the south ern coast of :Mindanao to the north of Luzon the mountains are disposed in a line with or parallel to the southern isthmuses. The whole interior is essentially mountainous, the Cordilleras ex tending north and south, their highest peaks rang ing from 3720 to 10,312 feet (Apo in Mindanao), with outliers and ramifications partly filling the gaps between the ranges. Narrow plains occur between the mountains and wider ones where the river valleys broaden near the coast, and are en riched with alluvial deposits. Mountains are the backbone of all the islands, though in Leyte there are no mountains of special prominence. Three ranges in Luzon and four in Mindanao are the dominating features in the topography of those largest islands. The wider plains are in the basins of the larger rivers of Mindanao and Luzon. Owing to the predominance of mountains, the area of tillable lands is not believed to be one-third of the total area.