332 NATURE AND MAN. 



the notice of both Arctic and Antarctic voyagers. And their ex- 

 quisitely sculptured cases, accumulating on the bottom, form a 

 siliceous " Diatom-ooze," which takes the place in higher latitudes 

 of the white calcareous mud, resulting from the disintegration of 

 foraminiferal shells. The foraminiferal ooze, moreover, generally 

 contains, in larger or smaller proportion, the beautiful siliceous 

 skeletons of Radiolaria ; and sometimes these were found to pre- 

 dominate to such a degree that the ooze mainly consisted of them, 

 in which case it was designated as radiolarian. As siliceous 

 skeletons are not — like calcareous — dissolved by deep-sea water, 

 those which fall down from the surface even upon the deepest 

 bottoms rest there unchanged ; and thus it happens that they are 

 found diffused through the red-clay deposits, and, at the greatest 

 depths, sometimes almost entirely replace them. Some of these 

 minute organisms were almost everywhere captured alive in the 

 tow-net; but, like the Diatoms, they commonly aggregate in 

 patches or bands, and this to such a degree as to colour the sea- 

 surface, the hue of their animal substance being usually red or 

 reddish-brown. Such patches are often seen in the neighbourhood 

 of the Shetlands, where they are designated by the fishermen as 

 " herring food." 



Thus, then, if we compare (i) the deposits now going on upon 

 the deep Oceanic sea-bed, (2) the sediments at present in course 

 of deposition in the shallower bottoms nearer land, and (3) the 

 materials of the sedimentary rocks of all geological periods, we 

 see that whilst there is a close correspondence between the second 

 and the third, the first difTers so completely — in most particulars 

 — from both the others as to be utterly beyond the range of 

 comparison with them ; the chief exception being presented by 

 those calcareous sediments which correspond with the various 

 Limestone formations intercalated among the sandstones and 

 clays that have had their origin in the degradation of the pre- 

 existing land. We now know for certain that the sands and clays 

 washed off the land — whether by the action of ice or river-waters 

 on its surface, or by the wearing away of its margin by the waves 

 of the sea — sink to the sea-bottom long before they reach the 

 deeper abysses ; not the least trace of such sed'unents having been 

 ajiywhere found at a distance from the coniitiental J>latforms. And 



