Considering that 'alpine-type' ultramafic rocks may be formed by differentiation of basaltic or andesitic magmas in pipes or chambers in the upper crust, the long narrow outcrops of such belts may mark the past positions of lines of volcanoes. Using the outcrops of Cretaceous ultramafics in Japan as an example, it is shown that two arcs can be traced, one mainly unserpentinized on the Pacific side, the other, on the inner side, serpentinized and with chromite content. It is suggested that the rocks of the outer arc were derived from a dry tholeiitic magma and those of the inner arc from an alkaline magma high in volatiles, subsequently serpentinized by autometasomatism and fumarolic activity. Prospecting along the inner arc by magnetic surveying could reveal hidden chrome ores.

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