242 PAPERS FROM THE DEPARTMENT OF MARINE BIOLOGY. 



and are thereby to be reckoned among the agents which break up calcium- 

 carbonate structures. 



Sizing is also of great importance, as it bears a most intimate relation 

 to strength of waves and currents. Material of different sizes is not dis- 

 tributed in a haphazard way, but is collected according to definite physical 

 laws in particular places. The relations of sediments to transporting agents 

 and conditions of deposition have not yet been adequately studied; in fact, 

 investigation of them is only now becoming definitely formulated; but enough 

 is at present known to justify the statement in the preceding sentence. 



The schedule of sizes here used is that of the Bureau of Soils, as Dr. 

 Cameron, of that Bureau, was so kind as to have the mechanical analyses 

 made there. This schedule meets the requirements of soil investigation, but 

 geologic work needs something different. At present facilities for the special 

 needs are not available, but it is the intention of the U. S. Geological Survey 

 to install a laboratory equipped for this particular work as soon as practicable. 

 The following statement gives the results of recent consideration of this prob- 

 lem by Messrs. M. I. Goldman, D. F. Hewett, G. S. Rogers, and E. W. Shaw: 



"Method of describing size of grains. — It was apparent that in order to give 

 readily a correct idea of the mechanical composition of sediments, and to be philo- 

 sophic, the system of sizing should have regular intervals. The great majority of 

 sedimentary rocks are composed of particles which have settled through some 

 moving fluid, and since the proportion of grains of various sizes depends on the 

 resultant sorting (the rate of fall of particles, other things being equal, varying with 

 the square of their diameter), the ratio of sizes should be constant and should 

 preferably be 4, 2, the square root of 2, or the fourth root of 2. Such a system of 

 sizing, in contrast with the prevalent ones in which variable ratios are used, has the 

 advantage that it does not give undue weight to a separate whose range in size is 

 greater than others. If a variable ratio is used, the result does not give so good an 

 idea of the composition of the sediment and is less significant as to its origin. It 

 the analysis is made by counting and not by sieving, the constant ratio is more 

 easily applied than a variable one, and the results of variable ratio analyses can be 

 converted more readily and accurately to a fixed ratio system than they could to some 

 other variable ratio system. 



"In ordinary work it seems probable that the ratio 2 analysis will give suffi- 

 ciently detailed information concerning the sediment. What will prove to be the 

 most satisfactory starting-point is not yet evident, but presumably it will be either 

 some point in the metric system, such as a centimeter, a millimeter, or a micron, 

 or the figure adopted by the Bureau of Standards for a 200-mesh screen, viz, 

 0.074 mm." 



It will be pointed out, in discussing the samples taken on the Murray 

 Island reef flat and behind the reef off Cocoanut Point, Andros Island, Baha- 

 mas, that there is very little material of silt and clay size. The explanation 

 of this condition demands consideration of the possibility of the removal of 

 fine material (of the size of silt and clay) by means of solution by sea-water, a 

 subject which will be discussed in its proper place (pages 265-268, and Dr. 

 Wells's paper, pages 316-318). 



