Sea-hottom procured hy Il.M.S. ^ Challenger.^ 293 



covering largo areas in the Atlantic, and met with also between 

 Kerguelen's Island and Melbourne. Into this red clay he describes 

 the Glubu/t-riiui-ooAe as graduating through the " grey ooze ; " 

 and he atlirins this transition to be essentially dependent on the 

 depth of the bottom. " Crossing," he says, " from these shallower 

 regions occupied by the ooze into deeper soundings, we Jind univer- 

 sally that the calcareous formation gradually passes into, and ia 

 replaced by, an extremely pure clay, which occupies, speaking 

 generally, all depths below 2500 fathoms, and consists almost en- 

 tirely of a silicate of the red oxide of iron and alumina 



The mean maximum depth at which the Glohigerina-oo'm occurs 

 may be taken at about 2250 fathoms ; the mean depth at which 

 we find the transition grey ooze is 2400 fathoms ; and the mean 



depth of the red-clay soundings is about 2700 fathoms 



We were at length able," he continues, " to predict the nature of 

 the bottom from tfie depth of the soundings ivith absolute certainty for 

 the Atlantic and the Southern Sea." And from these data he 

 considers it an indubitable inference "that the red clay is essentially 

 the insoluble residue, the ash, as it were, of the calcarepus or- 

 ganisms which form the Globiyerina-ooze after the calcareous mat- 

 ter has been by some means removed." This inference he considers 

 to have been confirmed by the analysis of several samples of 

 Globiyerina-ooiQ, "always with the result that^^a small proportion of 

 a red sediment remains, which possesses all the characters of the 

 red clay." Prof. "NVyville Thomson further suggests that the removal 

 of the calcareous matter may be due to the presence of an excess 

 of carbonic acid in the bottom-waters, and to the derivation of this 

 water in great part from circumpolar freshwater ice, so that, being 

 comparatively free from carbonate of lime, its solvent power for 

 that substance is greater than that of the superjacent waters of the 

 ocean. He might have added probability to his hypothesis if he 

 had cited the observations of Mr. Sorby as to the iucrease of sol- 

 vent power for carbonate of lime possessed by water under greatly 

 augmented pressure*. 



Greatly struck with the ingenuity of this hypothesis, I turned to 

 Prof. Wyville Thomson's tabular statement of the facts in detail, 

 and must own to a great feeling of surprise at the want of con- 

 formity of these details with the assertions of universality and 

 certainty of prediction which I have italicized in the above extracts. 



evidenced by the rounding into pebbles of what was elsewhere angular gravel. 

 But it is even more conclusively shown by a comparison of the two serial 

 soundings taken in the " cold area " (^Nos. 52 and (J4), which proves that the 

 glacial stratum /oM)S up a slope in the former position (just as the cold under- 

 stratum does in the Florida Channel), which it could not do unless it were in 

 movement. That we did not trace the outflow of this cold stream into the 

 great b:isin of the Atlantic, was simply, as I believe, because we were prevented 

 from ascertaining the bottom-temperature on the line which 1 expected that 

 flow to take after surmounting the ridge. 



* Proceedings of the Royal Society, vol. xii. p. 538. 



