149 



On the Estimation of the Numbers of Foraminifera found 



in Chalk. 



By M. C. Cooke, M.A., A.L.S., President. 

 {Read August 25th, 1882.) 



Having thought that it would be of advantage to place on record 

 the result of my calculations of the number of foraminifera found 

 in Kentish Chalk, and an opportunity now presenting itself of in- 

 troducing the subject, I may premise by stating, that the investiga- 

 tions were made some fifteen years since, although the details were 

 never printed ; and subsequent, but less complete, estimates lead me 

 to infer that the conclusions are substantially correct. 



In order to test the accuracy of other observers, as well as to 

 procure some new determinations of the number of organisms, ap- 

 proximately, to be found in the chalk, I took one ounce of chalk from 

 the jnt and washed away the lighter fragments by continued wash- 

 ings, until I obtained a sediment of nearly pure foraminiferous shells ; 

 half of this I cleaned as much as possible by boiling in caustic 

 potash, and ultimately found that I had enough material to mount 

 190 microscopical slides, each of which contained upwards of 1,000 

 jjerfect shells, so that in one ounce of chalk I had isolated 400,000 

 shells. Afterwards I washed another ounce more carefully, and 

 calculated that I had then obtained upwards of half a million of 

 entire shells, without reckoning the fragments which had been 

 washed away, or the thousands probably, that had been decanted 

 oft 1 with the water in 40 or 50 washings. Hence I am convinced 

 that I am very far short of the maximum when I name half a 

 million of Foraminifera as contained in every ounce of chalk from 

 that pit. The little lump of chalk which I procured for these 

 experiments weighed 16 pounds or 256 ounces, and consequently 

 contained the shells of one hundred and twenty-eight millions of 

 Foraminifera — a number easily named, easily written, but by no 

 means so easy to imagine ; a number which it would occupy a 



Journ. Q. M. C, Series II., No. 4. n 



