A light green rock occurring at Diphwys, hill, on the Harlech dome in northwest Wales, interbedded with the shales of the lower Cambrian Manganese Shale group, is suspected of being a spilitic type. Chemical data, calculated epinorm, and mode are given. The rock is not considered to be a metasomatized variety of the shales and it is also distinct from greenstone sills in the shales. It is thought, however, that it is the same age as these sills and was formed closer to the sea floor by basic magma penetrating wet unconsolidated sediments.

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