A detailed study of some bottom samples collected by P.S. Galtsoff during a survey of the Pearl and Hermes reef in 1930 has yielded some interesting quantitative data on modern tropical reefs. About half of the bottom material is composed of coralline algae remains; 17.8 per cent of mollusk shells: 16.6 per cent of coral fragments; and 6.3 per cent of foraminifera tests. The remaining 10.8 per cent is classified under eight other groups of constituents. The mechanical analyses reveal a symmetrical gradation of particle size ranging from coarse gravel near the inner edge of the reef to fine sand near the center of the lagoon. Comparisons of some organic ratios are made with shallow water marine deposits of southeastern Florida and the Bahamas.