The whole surface of the Philippines is essentially mountainous, the only plains that occur being alluvial districts at the river mouths and the spaces left by the intersection of the ranges.  The principal ranges have a tendency to run north and south, with a certain amount of deflection east and west, as the case may be, so that the orographic diagram of the archipelago, as a whole, has a similarity to a fan, with northern Luzon as its center of radiation.

While none of the mountain peaks greatly exceed 8,000 feet in height, Apo, in Mindanao, is over 9,000 feet; Halson, in Mindoro, is over 8,900 feet; and Mayon, in Luzon, over 8,200.  The latter is an active volcano, which has been the scene of several eruptions during the present century.  Extinct or active craters are relatively as numerous in the Philippines as in the eastern archipelago, and as a consequence of these subterranean forces earthquakes are frequent and violent.

In 1627 one of the most elevated mountains of Cagayan disappeared, and on the island of Mindanao, in 1675, a passage was opened to the sea and a vast plain emerged.  The more recent of the convulsions occurred in 1863 and in 1880.  The destruction of property was great, especially in Manila.

The general belief is that the Philippines once formed a part of an enormous continent from which it was separated by some cataclysm.  This continent probably extended from Celebes to the farthest Polinesian islands on the east, to New Zealand on the south, and the Mariana and Sandwich islands on the north.

These islands, according to Ramon Jordana, are divided into two volcanic regions, the eastern and the western.  The principal point is the volcano Taal, located in the northeastern portion of the province of Batangas.  It is situated on a small island in the center of the Bombon laguna, and has an altitude of 550 feet above sea level.  Its form is conical, and the rock is composed of basalt feldspar with a small quantity of augite.  The crater is supposed to be 232 feet deep.  Its sides are almost vertical, and there are two steaming lagunas at its bottom.

In the regions embracing the provinces of Manila, Bulacan, Pampanga, Tarlac, and Pangasinan the soil is mostly composed of clay containing remnants of sea shells, a circumstance which gives rise to the belief that the coast of Manila has risen from the sea in not so remote an epoch.  Smooth, dark gray tophus predominates; it forms the bed of the Rio Pasig, and rising forms hillocks in the vicinity of the city of Binangonan.  Farther on, trachyte and banks of conchiferous sand predominate.

The vast plain of Panpanga extends to the north of Manila Bay, to the south of which is situated Mount Arayat, of doleritic nature.