The geology of the Samoan archipelago is described, especially Tutuila where detailed work was done, and the geologic literature is reviewed. Tutuila was built by five volcanoes over two and possibly three parallel rifts trending N. 70° E. A caldera 6 miles across, formed by collapse of the top of the Pago volcano, is described, and its bounding fault is mapped. Pago Pago Bay is a drowned river valley cut by a stream along the curved base of the caldera wall. A thick series of tuffs and differentiated lavas partly filled the caldera. Several bulbous trachyte domes in a highly viscous condition pushed through these tuffs to the surface. One of them dragged upward with it for hundreds of feet a thick section of basalts. About 600 dikes and 40 faults are recorded. Several of the dikes are unusual; two contain oriented dunite xenoliths, and one contains bedded ash. Drilling shows that the fringing reefs of Tutuila are chiefly bedded calcareous sand and silt and not coral colonies.
Upolu is a deeply eroded mass of Pliocene lavas surrounded by a drowned barrier reef and partly buried by late Pleistocene and Recent lavas. Upolu was submerged so rapidly in late Pleistocene time that only narrow and interrupted fringing reefs grew on the steep shores of Pliocene rocks while a wide barrier reef developed on the gentle slopes of the flat-lying Pleistocene lavas. The absence of a continuous reef on Tutuila also is due to rapid submergence. Brief observations are recorded for the rest of the islands. The geologic histories of the various islands of the entire group are similar.