CHAP. X.] 



CONTINUITY OF THE CHALK. 



509 



whether the collection would be good or not before the dredge 

 came to the surface drawing his inference from the results of 

 his analyses of the gases of the bottom-water. In each case 

 his prediction was justified by the result. 



In the analyses made of the water in the cold area, and 

 generally in the third cruise, there appears, as might be 

 expected from the various currents, &c., a greater variation in 

 the results than in the other series. In the bottom and inter- 

 mediate waters the nitrogen appears to be rather in excess of 

 the average, and the carbonic acid has a large range of varia- 

 tion from 7'58 per cent, at Station 47 (540 fathoms, temp. 

 43-8) to 45-79 per cent, at Station 52 (384 fathoms, 30'6 

 Fahr.). The average of the surface-waters is much the same as 

 in the other parts of the cruise. 



It may be worth notice that in localities where the greatest 

 depth did not exceed 150 fathoms, the results of the gas-analysis 

 of bottom- and surface-water were frequently so nearly the same, 

 whatever the amount of animal life on the bottom, as to lead 

 to the supposition that there might be at that limit a sufficient 

 circulation, either of the particles of the water itself or of the 

 gases dissolved in it, to keep the gaseous constitution alike 

 throughout. The coincidence of this depth with the extreme 

 depth at which fish are usually found to exist in these seas is 

 suggestive. 



Organic matter. With a view to test the method of analysis 

 by permanganate of potash, two or three series of analyses were 

 lade where fresh and salt water mixed together, as in Killibegs 

 Harbour, Donegal Bay, &c. ; and the results in all cases justified 

 the expectation formed, that the amount of permanganate was 

 an index of the comparative purity of the water, both as regards 

 the " decomposed " and the " decomposable " organic matter. 



