MARINE DEPOSITS OF PATAGONIA 



181 



Fig. 2. Sieves stacked one above the other. 

 a, rubber ring; b, sieve plate. 



sieves from above while they were vigorously shaken. The material was thus quickly 



sorted into its components of various diameters down to 0-5 mm. in diameter, and at 



the same time it was thoroughly 



washed free from salt. The material 



finer than 0-5 mm. in diameter 



passed through the finest sieve at 



the bottom of the stack and was 



discarded. The sieves were then 



separated, and their contents dried 



and weighed. The figures obtained, 



in conjunction with those obtained 



from the levigator, were combined 



to show the percentage of the 



grades in the total sample. 



FINER MATERIALS 



A portion of the untreated 

 sample was placed in a sieve with 

 holes 0-5 mm. in diameter. This 

 was then thoroughly agitated in a 

 vessel of water so that all the material finer than 0-5 mm. in diameter passed through 

 into the vessel, and the coarser material of all grades over 0-5 mm. in diameter remained 

 in the sieve. The latter material was then well washed by a stream of water over the 

 vessel, dried and weighed. The finer material that passed through the sieve was then 

 levigated and thus separated into the two finest grades as described below. 



Borley ( 1 923) found that in North Sea deposits the separation of grades VIII (fine sand) 

 and IX (silt) could not be readily effected by decantation and so made use of a levigator. 

 The method of levigation was adopted in this investigation as being more accurate and 

 reliable than other methods. Schone (quoted by Borley) estimated that a vertical cur- 

 rent of water travelling at 7 mm. per second would separate spherical quartz particles 

 over o-i mm. in diameter from those under o-i mm. in diameter. Levigation by a 

 vertical current of this speed was found by Borley to give a very good degree of accuracy 

 (5 per cent) with North Sea deposits, and is the method adopted in this investigation. 

 The levigator used by Borley separated the material analysed into four grades, 0-2 mm. 

 in diameter and over, o-2-o-i mm. in diameter, 0-1-0-05 mm. in diameter, and 0-05 mm. 

 in diameter and under. The first two of these grades taken together formed grade " fine 

 sand" and the last two the grade "silt". The levigator used in this investigation separ 

 ated the material only into the two grades fine sand and silt, containing particles over 

 o-i mm. and below o-i mm. in diameter respectively. 



Apparatus 

 The levigator devised for use in this investigation is a development of that used by 

 Borley (1923), which in turn was modified from that of Schone. 



