Mountain building is still in progress and evidence of the recent vertical movement of various portions of mountain mass can be measured. The archipelago is the crumpled edge of the Asiatic continental platform. However, the structure is a complex one, for besides faulting and folding, volcanic actions of diverse kinds have taken place. Prior to the Tertiary period there seems to have been a period of intense deformation, when some of the schists were formed. Toward the end of the Miocene period, the Vigo and Batan formations were generally folded, in some places being intensively crumpled and faulted. Another less pronounced period of folding occurred in the Pliocene or early Pleistocene period. The islands were subjected to a great uplift during the Pliocene, Pleistocene and recent periods. A striking similarity exists between the formation in the Philippines and the western coast of America. Practically all the rock types are found in the islands, as well as areas of coral limestone.
The volcanoes are the most conspicuous feature of the landscape, but there is less volcanic activity in the whole group than in a single Hawaiian island. However, all gradations of volcanoes can be seen, from the almost perfect cone of Mayon to old, worn down volcanic stocks whose present form gives little indication of their origin. The several distinct volcanic areas are that of south-western Luzon (the provinces of Rizal, Cavite, Laguna, Batangas and a portion of north-western Tayabas) ; that of south-eastern Luzon; the mountain region in Camarines Norte ; Negros ; Mindanao; Jolo ; and a few others. Seismic disturbances are common.
The mean annual temperature is 26.9° C (80.4° F). There are seven months (April to October) with a mean monthly temperature of 26.9° to 28.1° C, and five (November to March) with 25.4° to 26.5° C. May is the warmest and January the coolest month. During the period 1903-18, the highest absolute temperature for Luzon was 42.2° C and the lowest, 12.1° C, and the highest and lowest for the Bisayas and Mindanao, 38.2° C and 13.3° C respectively. The Philippines would have little dif ference of climate if the rainfall were homogeneous. There are three types of climate The first is characterized by two pro nounced seasons, dry (with less than 2 in. of rainfall) in winter and spring and wet in summer and autumn. Cyclonic or summer rainfall prevails. This type occurs in the western part of Luzon, Mindoro, Negros and Palawan, and the western and southern parts of Panay. The second type is characterized by no dry season, has a pronounced maximum of rainfall in winter, and prevails along the eastern part of the archipelago from about the latitude of Manila southward, or in south-eastern Luzon, Samar, eastern Leyte and in Surigao Province, Mindanao. The
third or intermediate type has no very pronounced rain period and a short dry period of only one to three months. The average annual rainfall for the archipelago (whose annual mean varies between 181 and 38.9 in.) is about 93.1 inches. The subprovince of Benguet averages the heaviest precipitation—over 355 in. in 1911. The average for the whole archipelago shows 159 rainy days. The floods are usually caused by typhoons. Severe droughts occur at times especially in the west—two remarkable drought periods being those of 1911-12 and 1914-15. In terms of the rainfall there are said to be three seasons : the dry (January to May) ; the rainy (June to October) and the relatively dry (No vember to December).
About
species of flowering plants and ferns have been found. The flora is essentially Malaysian, but con tinental (Himalayan) elements occur in the mountains of north ern Luzon, while a few Australian types are found at various altitudes. There is a small percentage of endemic genera and a very high percentage of endemic species (about 6o%). The dominant forest species are usually endemic. Among the grasses of the grasslands are lalang (cogon), and other coarse grasses. The vegetation of the coastal regions (including the mangrove swamps) is practically identical with that of similar areas through out the Malay archipelago. Considerably more than half the land area of the islands is covered with forests. There are many valu able hardwoods as well as many of the softer variety. About 1,000 different woods have a commercial value, among which may be noted apitong, camagon, ipil, lauan, mayapis, molave, narra, tan gile, tindalo and yacal. Some timber trees attain a huge size which makes them especially adaptable for tables and other large pieces of furniture. There are many useful kinds of palms, including the coconut palm, from which the Filipino obtains food, clothing, fibre for export, coconut oil, copra, building material and other products. Useful bamboos and rattans abound. About 900 species of orchids occur, some of them very rare. Several hundred fibres are known, among them the aback from which manila rope is made. The forests also yield gums including rubber, various fruits, nuts, spices and drugs, oils and other useful substances.