BOTTOM SAMPLES, MURRAY ISLAND, THE BAHAMAS, AND FLORIDA. 241 



according to their source. MoUusks and stony corals, for instance, contain 

 almost no MgCOs, whereas certain foraniinifera contain from 9 to 1 1 per cent, 

 and coralline algae sometimes contain as high as 20 per cent. Later in this 

 paper the chemical composition of the deposits as ascertained by analysis will 

 be compared with the chemical composition as deduced from the percentage of 

 the respective organic ingredients forming them. This may be said here: if 

 a sand in a coral-reef area contains 7.5 per cent of MgCOs, probably not over 

 50 per cent of the deposit is of coral origin (see analyses of corallinaceous 

 algae on p. 248); and as moUusk shells and Halimeda contain almost no 

 MgCOa, and are generally present in coral areas, the probability is that less 

 than 50 per cent of the deposit is coral. 



SIZING OF SEDIMENTS. 



The mechanical condition of a sediment is important in many particu- 

 lars, for from it a number of deductions may be made. The particles in 

 bottom samples usually fall into two categories: (i) those which preserve 

 their original form, for example, the shells of many foraminifera, alcyonarian 

 spicules, etc.; (2) particles which have a secondary form, resulting from (a) 

 disintegration of the body of which it originally formed a part, (b) secondary 

 aggregation of particles, as in oolite grains. 



Agencies causing disintegration and reduction of size need brief consider- 

 ation. These are of two classes: (i) inorganic; (2) organic. The inorganic 

 agencies causing disintegration are waves and currents. Waves by their 

 impact break structures or reduce the size by hurling fragments one against 

 another. Both waves and currents reduce the size of particles by the attrition 

 of one against another. 



The numerous organic agencies which cause disintegration have been 

 investigated by many students, among whom are Duerden, Stanley Gardi- 

 ner, Wood Jones, and myself. Among the disintegrating agents are boring 

 algae, sponges, worms, moUusks, and echinoids. These all render calcareous 

 structures less able to withstand the effects of waves and currents. Gardi- 

 ner has properly emphasized the importance of sand-feeding organisms in 

 the production of silt. I have made a number of similar observations on 

 echinoids and holothurians in the Bahamas and in Florida. Some worms, 

 and probably other organisms, also tend to reduce the size of particles. 

 Unfortunately the amount of work accomplished by these agents has not 

 been evaluated. It is a difficult task, and it will probably be some time 

 before the order of magnitude may be ascertained. I have weighed the 

 inorganic content of the guts of a number of holothurians and echinoids, and 

 tried to ascertain the rate at which some passed sand, but the results seem 

 unsatisfactory, as so many factors are unknown that no reliable estimate of 

 effect is at present possible. The subject seems to me of sufficient importance 

 to warrant special investigation. According to Darwin, H. O. Forbes, and 

 Wood Jones, two species of Scarus, a genus of fishes, browse on living coral 



