346 GEIKIE 



sandy and pebbly material as we find so largely among the 

 sedimentary rocks of the land, wide tracts of the sea- 

 bottom at great depths are covered with various kinds of 

 organic ooze, composed sometimes of minute calcareous fo- 

 raminifera, sometimes of siliceous radiolaria or diatoms. 

 Over other areas vast sheets of clay extend, derived ap- 

 parently from the decomposition of volcanic detritus, of 

 which large quantities are floated away from volcanic 

 islands, and much of which may be produced by submarine 

 volcanoes. On the tracts farthest removed from any land 

 the sediment seems to settle scarcely so rapidly as the 

 dust that gathers over the floor of a deserted hall. Mr. 

 Murray of the Challenger staff, has described how from 

 these remote depths large numbers of shark's teeth and ear- 

 bones of whales were dredged up. We cannot suppose the 

 number of sharks and whales to be much greater in these 

 regions than in others where their relics were found much 

 less plentifully. The explanation of the abundance of their 

 remains was supplied by their varied condition of decay 

 and preservation. Some were comparatively fresh, others 

 had greatly decayed, and were incrusted with or even com- 

 pletely buried in a deposit of earthy manganese. Yet the 

 same cast of the dredge brought up these different stages 

 of decay from the same surface of the sea-floor. While 

 generation after generation of sea-creatures drops its bones 

 to the bottom, now here, now there, so exceedingly feeble 

 is the rate of deposit of sediment that they lie uncovered, 

 mayhap for centuries, so that the remains which sink to-day 

 may lie side by side with the mouldered and incrusted bones 

 that found their way to the bottom hundreds of years ago. 



Another striking indication of the very slow rate at which ,' 

 sedimentation takes place in these abysses has also been 

 brought to notice by Mr. Murray. In the clay from the 

 bottom he found numerous minute spherical granules of 

 native iron, which, as he suggests, are almost certainly of, 

 meteoric origin fragments of those falling stars which, 

 coming to us from planetary space, burst into fragments 

 when they rush into the denser layers of our atmosphere. 

 In tracts where the growth of silt upon the sea-floor is 

 excessively tardy, the fine particles, scattered by the dissipa- 



