Vol. 51.] THE CHEMICAL COMPOSITION OE OCEANIC DEPOSITS. 327 



W. X. Y. Z. 



Organic matter, etc '57 "50 - 10 - 40 



Calcium carbonate 9520 97-02 96-15 94-94 



Calcium sulphate -17 "41 -29 "31 



Magnesium carbonate "68 — 134 "25 



Colloid silica -65 -45 -35 -48 



Quartz "55 "30 "15 '10 



Clay, alumina, and magnesia ... 166 1*03 100 2'95 



Iron peroxide '40 "26 -28 '40 



Manganese peroxide *28 *05 '23 "27 



Alkalies -16 -04 -27 '23 . 



100-32 100-06 100-16 10033 

 W is a sample from the lower part of the Melbourn Kock near Okeford 



Fitzpaine, Dorset. 

 X is a sample from the upper part of the same rock. 

 Y is Chalk about 60 feet above that rock at Hitchin. 

 Z is Chalk Rock with glauconite-grains from Aston Rowant, near Watlington in 



Oxfordshire. 



The similarity between these and the two oozes marked B and C 

 on p. 322 is very great. It must be remembered that, when the 

 adherent sea-salts are allowed for, the proportions of calcium car- 

 bonate in these amount to 93*46 and 92 respectively. The Chalks 

 differ chiefly in having less peroxide of iron than the oozes. The 

 small amount of quartz, both in Chalk and ooze, is especially 

 noticeable. 



VI. Miceoscopical Stetjctt/re oe Oozes. 



While the chemical composition of the thre'e samples of ooze is 

 very similar to that of certain kinds of chalk, both Tertiary and 

 Cretaceous, the microscopical structure exhibits some differences. 



The two Atlantic oozes, B and C of p. 322, have, in the mass, a 

 distinctly granular appearance which is evidently due to the great 

 abundance of robust, thick-walled tests of foraminifera. In a 

 thin slice of the dried material Globigerince and other forami- 

 nifera are seen to occupy the greater part of the field ; but the 

 intervening matrix has the characters of chalk, consisting almost 

 entirely of fine calcareous matter in which minute foraminifera 

 and single-celled calcareous spheres with thin walls are abundant. 

 These spheres are much more abundant in the ' Challenger ' sample 

 (B) than in the ' Buccaneer ' sample (C). 



We are indebted to Mr. William Hill, F.G.S., for the following 

 observations on these oozes when examined with high powers : — 

 ' The fine matrix shows no definite structure when seen under high 

 powers, but the idea gradually possesses one that most of it must be 

 foraminiferal debris. In the ' Challenger ' specimen there are 

 multitudes of exceedingly minute spicule-like bodies, club-shaped, 

 with three hooks at the thinner end ; they disappear when treated 

 with acid. Coccoliths are present, but are small and thin. There 

 is a fair number of radiolaria and diatoms in both specimens. A 

 few grains of pumice also occur in both, but otherwise there is very 

 little inorganic material.' 



The Pacific ooze (A) has a less granular and a more chalky 

 appearance. In section Globigerince are seen to be abundant and 



