378 ON THE FAUNA AND BOTTOM-DEPOSITS NEAR THE 30-FM. LINE 



The method of procedure which has been adopted will now he 

 described. 



Obtaining the Sam2)l€. In order to get a correct idea of the nature 

 of the deposit it is not sufficient to take a sample of so much of it as 

 happens to come up in the ordinary dredge. Samples obtained in this 

 way have lost much of the finer portions of the deposit, and generally 

 give a very incorrect idea of its nature. The samples, the composition 

 of which is here described, have always been taken with a moderate- 

 sized dredge (about 2 ft. 8 in. long),* fitted with a bag of stout canvas 

 inside the ordinary net. At the end of a haul of the ordinary dredge 

 it was our custom to steam back a little way over our course and then 

 work this dredge with the canvas bag, leaving it down only for two or 

 three minutes. In this way a good sample of the deposit was obtained, 

 and the haul often yielded interesting species not obtained in the usual 

 way of dredging. 



Examination of Samples for Texture. The samples were brought back 

 to the Laboratory, and for their examination a series of sieves made of 

 zinc and with perforated zinc bottoms was employed, through which 

 the samples were washed. The sieves had a diameter of 15 centimetres 

 and six of them were used, the sizes of the (circular) perforations being 

 15 mm., 5 mm., 2*5 mm., 1"5 mm., 1 mm., and 0*5 mm.f The material 

 passing through the 0"5 mm. sieve was further subdivided into two 

 parts, one of which was regarded as fine sand and the other as silt. 

 This separation was made by stirring the material up with a considerable 

 quantity of sea-water, allowing it to settle for one minute, and then 

 pouring the water off. The process was generally repeated a second 

 time, the material remaining in suspension being regarded as silt, whilst 

 that which settled in one minute was regarded as fine sand. 



In this way eight grades of material were obtained altogether, and 

 it was found that these could conveniently be distinguished by the 

 following nomenclature : — 



I. Stones. All inorganic material which will not pass through 

 15 mm. sieve, 

 11. Coarse Gravel. Material left on 5 mm. sieve. 



III. Medium Gravel „ „ 2-5 mm. „ 



IV. Fine Gravel „ „ 1*5 mm. „ 

 V. Coarse Sand „ „ 1 mm. „ 



VI. Medium Sand ,, „ 05 mm. „ 



• The size of the dredge is important, as a smaller and lighter one would probably not 

 dig so deeply into the deposit. A strong net is retained outside the canvas bag in order 

 to take some of tlic strain when the dredge is brought up. 



t Perforated zinc with holes of these diameters can be oljtained from Messrs. J. 

 Stauiar & Co., Manchester Wire Works, Manchester. 



