A petrologic study of the subsurface clays and marine limestones from the Castle Harbour area; the weathered and water-worn volcanics from the Gibbs' Hill bore hole; a sample from the Prospect Military Well; a conglomerate sample from the Hard Bermuda Limestone, all from the Bermuda Islands, is described.

All allogenic mineral species found in the sediments are also found in the volcanics; similar minerals were reported by Sayles (1931) from the surface eolianites and soils of the island. The volcanics carry minerals not found in the sediments, but these are minerals whose stability is lowest under weathering conditions.

A chemical as well as a textural and a petrographic analysis is given of the most interesting and important sediment, the Primary Red Clay. This clay is a type of lateritic soil, the result of sub-aerial, tropical, humid, weathering of the igneous rocks of the island. A distinctive feature of this clay is the high content of phosphatic material. Part of this phosphate is in the form of collophanite; the rest seems to be an actual constituent of the clay. This phosphate was added to the clay after the clay was formed, when it was covered by the marine waters of a lagoon.

No exact age for the clays and marine limestones was reached, but they would appear to be of late Tertiary age.

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