Te 9 samples it was less than 5%. Silicification appears to be associated particularly with the yellow-green algae, the Chrysophyta™”, Thus the residue might derive, in part, from epiphytic diatoms.“!) The insoluble residues might also represent mechanically en‘ trapped contamimants. It would be expected that those algae which have a mat- or sponge-like struc- t in und ore, was ave ‘ac wn ted the shows that, depending on collection location, the sand can consist of from 60% to 90% of calcium carbonate and other acid soluble components. It may be further noted from the data in Table 2 that the concentra- tions of radium, uranium, and calcium in sand are very similar to those reported for the samples of Halimeda opuntia or Penicillus capitatus (Table 1 also}. This is consistent with the observations that ture, and which grow where there is considerable sus- pended material of land origin, would show the greatest degree of such entrapment. It is of interest, therefore, that the spongy Aurainvillea rawsoni shows the greatest degree of variation in insoluble residue, its soluble the sands in the region of collection are made up largely of the mineral residues of calcareous orga- nisms, particularly Halimeda."*) Thus, it is possible that calcareous contamination might have contributed to the extent of variance found for some of the analy- fraction varying between 0.98 and 0.48 for different samples. Laurencia papillosa and Dictyota divaricata ses. For those species where samples were collected in also show a large variation in their soluble fractions. The degree of contamination by silica would also depend on the nature of the bottom sediments where the algae are growing. The importance of this factor may be inferred from the variability in the values of sufficient number in 1961 and again in 1967 or 1968, there were no significant differences between the two groups in their concentrations of calcium, radium, uranium or nitrogen. An insufficient number of sam- ples from each location precludes an evaluation of the analyses for calcium and insoluble matter in the samples of sand from different locations (Table 2). the effect of the environment at each collection site or Even the limited sampling represented by Table 2 °-20/ ' of location dependent differences for the various spe- O ] oO O CHLOROPHYCEAE O RHODOPHYCEAE & PHAEOPHYCEAE 0.15 A o — SS apa Qo oO A _ oO E an = 2 o = 0.10 ole A A oO A A = oS A a oc oO a AA A A Do oOo Oo o.oo a oo o| 0 A = Oo oO CA Oo o oO Oo © 4o oOo — Oo A 0.05f— A oO A o oO oO 0 co oO oO ° a J 0.1 ] 0.2 | 0.3 0.4 CALCIUM (qm/gm) Fig. 4.—The variation in the concentration of radium with the degree of calcification of the algae