14 THE VOYAGE OF H.M.S. CHALLENGER. 



HYPOTHESIS ON THE FOEMATION OF BANKS AND DEPOSITS OF 



DIATOMS. 



In 1876 Mr Murray read before the Eoyal Society of Loudon a Preliminary Report ' 

 on Oceanic Deposits, appended to 'which is a map in which the existence of a bank 

 formed almost exclusively of the siliceous remains of Diatoms is indicated ; and further 

 details have been more recently given by Mr Murray in the Narrative of the Cruise. 2 This 

 bank extended between lat. 60° 52' S., long. 80° 20' E., and lat. 53° 55' S., long. 108° 35' E. 

 It therefore measured not less than 1700 miles in length. 



Such accumulations lead one to inquire how it is that Diatoms which vegetate in all 

 seas should accumulate in this manner in particular localities which are in all probability 

 few in number, since similar formations were not recorded by the naturalists on board the 

 Challenger in any other region. 



A Diatom as soon as it ceases to live is no longer supported by the globules of oxygen 

 which it evolves and which adhere to it, but is left to the mercy of the waves, and in 

 virtue of its own weight, it sinks to the bottom, which it will reach in a relatively short 

 space of time, notwithstanding the disturbing tendencies exercised by oceanic currents, as 

 the density of the water is but little affected by the constantly increasing pressure. It is, 

 moreover, not improbable that the cooling and consequent descent of the surface waters in 

 the neighbourhood of the Antarctic ice, facilitate to some extent the sinking of the dead 

 frustules and their accumulation on the bottom in these regions. 3 



It is a matter of very considerable importance from a geological point of view to 

 determine exactly the conditions under which diatomaceous deposits like those discovered 

 by the Challenger are laid down, inasmuch as what is now taking place in the Antarctic 

 supplies a clue to the elucidation of the phenomena which took place in remote epochs 

 when similar marine diatomaceous banks were laid down. 



Italy affords a very interesting example of such formations. For several years it has 

 been known to the scientific world that, in the celebrated sulphur mines of Sicily, siliceous 

 schists occur which not unfrequently contain specimens of fish. These schists have been 

 found to consist very largely of Diatoms and Radiolaria, so that their marine origin cannot 

 be doubted. 



More recently the curiosity of palaeontologists has been attracted to a locality in 



Central Italy called Mondatino, in Montefeltro, where, among the strata of siliceous marl, 



a number of fishes were found in schists, which, like the former, were extensively formed 



of the remains of marine Diatoms and of the skeletons of Eadiolaria and Polythalamia. 



This formation lies along the spurs of the Apennines, extending on the one side as 



1 Proc. Roy. Soc. Land., vol. xxiv. p. 471, 1876. " Narr. Chall. Exp., vol. i. p. 432. 



3 Exploration of the Antarctic Regions, Scottish Geographical Magazine, vol. ii. p. 537. 



