us 



water. Professor Wyville Thompson in the cruise off the north coast 

 of Scotland procured no less than 40 vitreous sponges in one haul of 

 the dredger. 



And now he would call particular attention to some extracts from 

 a paper by the same distinguished man on " The Challenger" expedi- 

 tion (vide Nature of December loth, 1874). They were doubtless 

 aware that in the soundings of that expedition the calcareous 

 Globigerina ooze was found to disappear at an average depth of 

 1,800 fathoms and to be replaced at greater depths by a red clay made 

 up principally of silicates of iron and alumina. " There seems to be 

 no room left for doubt that the red clay is essentially the insoluble 

 residue ; the ash as it were of the calcareous organism which forms the 

 Globigerina ooze after the calcareous matter has been by some means 

 removed. In this ooze siliceous bodies, including the spicules of 

 sponges, the spicules and tests of radiolarians, and the frustules of 

 diatoms occur in appreciable proportions ; and these also diminish in 

 number, and the more delicate of them disappear in the transition 

 from the calcareous ooze to the red clay. All sea-water contains a 

 certain proportion of free carbonic acid, and Mr. Buchanan believes 

 that he finds it rather in excess in bottom water from great depths. I 

 have already alluded to the large quantity of nodules of the peroxide 

 of manganese which were brought up by the trawl from the red clay 

 area on the 1 3th of March. Any large organic body, such as a shark's 

 tooth, that may happen to be in the ooze is more or less completely 

 replaced by manganese ; and any inorganic body, such as a pebble 

 or a piece of pumice, is coated with it. . . . At Station 160 the 

 trawl brought up nearly a bushel of nodules from the size of a walnut 

 to that of an orange." And this they should remember notwithstand- 

 ing that the highest analysis of manganese in the ashes of sea- weed 

 gave only 4 per cent. 



No one could speak of the depositions of silica without taking 

 into account one of the most striking examples of it — a flint nodule ; 

 and since science, like Charity, should begin at home, it behoved them 

 especially to take them into account. Chemically, flint consisted of 

 about 97 per cent, of silica, i per cent, of alumina and oxide of iron, 

 about 2 per cent, of water, and commonly carbonaceous colouring 

 matter. Of silica, a certain (varying) portion was soluble or opal- 

 silica. Physically, flint might be briefly defined as a hard, semi-opaque. 



