Structural and physiographic elements of north Luzon include (from east to west) the Sierra Madre and the Caraballo mountains to the south, composed essentially of andesitic rocks, with early Tertiary metavolcanic and metasedimentary beds in marginal areas; the Cagayan valley, an intermontane structural basin containing folded and faulted late Tertiary eugeosynclinal deposits; the central cordillera, consisting of intermediate to mafic plutonic rocks, and like the Sierra Madre, with great thicknesses of basalt and graywacke in marginal zones; and a structural basin whose western margin lies beneath the South China Sea and which constitutes a northern extension of the Central Valley of Luzon.

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