In the islands north and east of Australia, as far as the Carolines and Fiji, are occurrences of ancient metamorphic rocks like schist, gneiss, slate; sediments such as coal, clay, and sandstone; intrusive granite and siliceous eruptive rocks such as andesite, all of which are characteristic of continents. The volcanic islands in the north Pacific short of the Aleutians, and the eastern Pacific to the mainland, are predominantly composed of dark and heavy basalt, which is thought to underlie the broad, deep ocean there. The name andesite line or sial line has been given to the boundary between the islands whose volcanoes extrude andesite and other sial rocks, which are on the west side of the Pacific, and those in which basalt is the chief lava in the eastern and mid-Pacific. In the opinion of many geologists the andesite line represents the farthest outer limits of a continental mass that once extended into the Pacific area. This hypothetical southern continent is supposed to have connected Australia, New Zealand, Fiji, the Solomons, New Hebrides, New Caledonia, and New Guinea with Asia and may have begun sinking in the late Mesozoic era; possibly the process continued to affect the fragments of the continent through the Cenozoic to the present. The andesite line is the boundary between the continental blocks of Asia and Australia and the Pacific basin.
The andesite or sial line follows the eastern edge of New Zealand, Fiji, Solomons, Bismarck Archipelago, New Guinea, Yap, Marianas, Japan, Kamchatka, and south of the Aleutians. Authorities differ as to whether Truk is west or east of the sial line. The island arcs west of the andesite line, and the Pacific coast ranges of North and South America, show marked deformation of the bedrock, which is folded, faulted, and frequently strongly metamorphosed. The real Pacific basin lies within the horseshoe rim of the mountain uplifts whose deformation began in the Tertiary and in some sections has continued to the present.