GEOGRAPHICAL EVOLUTION. 553 



nifera, sometimes of siliceous radiolaria or diatoms. Over other areas 

 vast sheets of clay extend, derived apparently 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 sedi- 

 ment 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 sharks' teeth 

 and ear-bones of whales were dredged up. We can not suppose the 

 number of sharks and whales to be much greater in these regions than 

 in others where their relics were found much less j)lentifully. 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 

 deeply 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 th^ remains which sink to- 

 day may lie side by side with the moldered and incrusted bones that 

 found their way to the bottom hundreds of years ago. 



Another striking indication of the very slow rate at which sedi- 

 mentation takes place in these abysses has also been brought to notice 

 by Mr. Murray. Among 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 tai'dy, the fine par- 

 ticles, scattered by the dissipation of these meteorites, may remain in 

 appreciable quantity. In this case, again, it is not needful to suppose 

 that meteorites have disappeared over these ocean-depths more numer- 

 ously than over other parts of the earth's surface. The iron granules 

 have no doubt been as plentifully showered down elsewhere, though 

 they can not be so readily detected in accumulating sediment. I know 

 no recent discovery in physical geography more calculated to impress 

 deeply the imagination than the testimony of this meteoric iron from 

 the most distant abysses of the ocean. To be told that mud gathers 

 on the floor of these abysses at an extremely slow rate, conveys but a 

 vague notion of the tardiness of the process. But to learn that it 

 gathers so slowly that the very star-dust which falls from outer space 

 forms an appreciable part of it, brings home to us, as hardly any- 

 thing else could do, the idea of undisturbed and excessively slow accu- 

 mulation. 



From all this evidence we may legitimately conclude that the pres- 



