i8o 



DISCOVERY REPORTS 



ANALYSIS OF THE SAMPLES 



For the sake of uniformity, and to afford comparison with other similar investigations, 

 the standard grades of texture used by Allen (1899) and Borley (1923) for the analysis 

 of bottom samples were adopted. 



The coarser materials were passed through sieves, and the finer materials were 

 levigated, so that they were divided into grades as follows : 



Material over 15 mm. in diameter. 



,, 10 mm. and under 15 mm. in diameter. 



5 - IO 



2-5 



i-o 



°-5 

 o-i 



5 

 2-5 



i-o 

 °-5 



Material under o- 1 mm. in diameter. 



Borley 's (1923) method of separating the grades consisted in twice washing the 

 sample to free it from salt, and drying it. A portion of this was then weighed, sifted, 

 usually in water, and the grades produced dried and weighed. A portion of the material 

 passing through the finest sieve was then weighed, moistened and levigated. The re- 

 sulting grades, except the finest, were dried and weighed. The finest material was 

 rejected and its weight arrived at by subtraction. 



In this investigation a technique that avoids the repeated dryings and weighings was 

 devised. The sifting and levigating is combined with the washing, and the grades are 

 weighed once only, and by addition give the total weight of the sample examined. 



COARSER MATERIALS 

 Apparatus 

 A series of sieves made of perforated zinc with circular holes 15, 10, 5, 2-5, 1-5, i-o, 

 and 0-5 mm. in diameter were used for separating grades I-VII from each other and from 

 the finer materials. The diameter of the sieve plates was 15 cm.; the sides were 7 cm. 

 high and tapered slightly so that the mouths of the sieves were 16 cm. in diameter. The 

 sieve plates were 1 cm. from the bottom of the sides, which thus rose 6 cm. above them. 

 Round the mouth of the sieves was fitted an india-rubber ring with a groove in it so 

 that it slipped on to the rim. The sieves could be stacked one above the other, each 

 slightly projecting into the one below, the india-rubber rings on the rims making a 

 watertight joint between each (Fig. 2). 



Method of use 

 The sample to be analysed was thoroughly stirred and a portion was placed in the 

 upper one of the stack of sieves. A strong stream of water was then directed through the 



