FROM THE EDDYSTONE GROUNDS TO START POINT. 381 



Coccoliths can be readily obtained in large numbers by shaking up 

 a small quantity of bottom-deposit with sea-water, pouring off the 

 muddy water, and allowing it to settle. I have found them to be much 

 more numerous in samples taken from below 15 fathoms than in those 

 obtained from the shallow waters of Plymouth Sound, although a few 

 can generally be found in the latter. These organisms must now there- 

 fore be regarded as very abundant in the sea, and their importance in 

 the economy of many marine animals must be great, since they furnish 

 a source of food-supply containing carbonate of lime. 



Determination of the Amount of Carbonate of Lime in the 



SAMPLES OF Bottom-deposit, and Description of their 



Geological Features. By E. H. Worth. 



" 1. Average Grade of Sample. — The sands and gravels had already been 

 sieved and graded before their examinations for geological purposes and for 

 the determination of carbonate of lime. It appeared evident, however, that as 

 a ready means of determining the relative texture of a whole sample it would 

 be necessary to compress the information contained in the table which gives 

 the percentage of each grade in each sample (Table II., p. 525). 



" The method adopted is founded on the determination of the average grade 

 of the whole sample. The figures I., II., III., etc., having reference to grades 

 left in a sieve of 15 mm., 5 mm., 2*5 mm., etc., are purely conventional, but 

 may be conveniently adopted for the purpose of shortly stating the average 

 grade of any sample. The actual method followed was to multiply each per- 

 centage in the table by the conventional number attached to its grade, add 

 together all the figures so obtained, and divide by 100 ; the result has been 

 described as the ' average grade ' of each sample, and affords a ready means 

 of comparing the texture of any two deposits. Close comparison still of 

 necessity requires a reference to the detail on which this average is based. 

 As an instance of the method the determination of the ' average grade ' of 

 sample 83 is appended : — 



394-6-^-100 = 

 3 946, the 'average grade' of sample 83. 



"Averaged in this manner the samples fall naturally into two divisions; the 

 one division lies between the grades 3-6 and 4 "6, and the (>thcr between the 



NEW SEKIES. — VOL. V. NO. 4. 2 C 



