ON KOCK EEMAINS IN THE BED OF THE ENGLISH CHANNEL. 109 



and fragments of Polyzoa, especially Ccllaria and Cdlepora, rendered 

 the accurate grading of them a difficult matter, owing to the repeated 

 breaking of the more delicate fragments in the sifting process, so that 

 some small excess error must be allowed for throughout in the direction 

 of the finer grades. The percentage of Carbonate of Lime present in 

 the material above and below the dimension of 0*5 mm. has been 

 determined as shown in Table 11. The proportion of this due to 

 inorganic matter is so slight that the percentage may be treated as 

 representing entirely organic remains. For convenient comparison of 

 the samples an " average grade " is added in each case. This method 

 of averaging the samples, which was used by Mr. Worth in estimating 

 the texture of bottom-deposits of the Start to Eddystone G-rounds,* 

 consists in multiplying each grade - percentage by its conventional 

 number (15 mm. + = I, . . . /. 0'5 mm. = VIII,) and then dividing the sum 

 of the products by 100, the quotient being the average grade of the 

 sample. It is an interesting point to observe that in these samples, 

 as is shown in Table II, there is a distinct tendency for the average 

 grade to decrease, i.e. for the texture to become coarser, with the 

 increase of distance outwards, as far as (75) at 38 miles, beyond which 

 an increase occurs up to (79) at 48 miles. If these few samples be 

 grouped together on broad lines of 10-mile intervals on a S.W., Mag. 

 bearing, the combined averages appear as follows : — 



5-10 miles . 

 10-20 „ . 

 20-30 „ . 

 30-40 „ . 

 40-50 „ . 



Sample (50 ), falling on the 30-mile point, is included in both 

 intervals between 20 and 40 miles. 



While this method of averaging is useful as indicating the com- 

 parative texture of the samples, it is open to the objection that the 

 inclusion of the coarser grades may unduly influence the result in the 

 way of obscuring the finer ones. If the coarser grades be disregarded, 

 and only those below and including 1 mm. -l- be considered, the average 

 percentage of material within this range for the samples grouped in 

 the same manner as before works out as follows : — 



5-10 miles .... 95-0 per cent. 



10-20 „ .... 77-1 „ 



20-30 „ .... 76-2 „ 



30-40 „ .... 52-1 „ 



40-50 „ . . . . 41-6 „ 



* Journal of the Marine Biological Association, Vol. V, p. 381. 



