OllIGIN OF OCEANIC AREAS 1 093 



vesicular lava, which, after long floating and dispersion by surface- 

 drift or ocean-currents, have become water-logged and have sunk to 

 the bottom. As no ordinary silicious sand is found anywhere save 

 in the neighbourhood of continents and continental islands, and as 

 almost all oceanic islands are either of volcanic origin or coral atolls, 

 this almost universal absence of any trace of submerged continental 

 land over the great oceanic area affords strong confirmation to the 

 belief that the sedimentary rocks which form the existing land were 

 deposited in the neighbourhood of pre-existing land, whose degrada- 

 tion furnished their materials ; and suggests that the original 

 disposition of the great continental and oceanic areas was not very 

 different from what it now is. 1 Further, the microscopic examination 

 of these oceanic sediments reveals the presence of extremely minute 

 particles, which seem to correspond in composition to meteorites, and 

 which there is strong reason for regarding as ' cosmic dust ' pervading 

 the interplanetary spaces. Thus the application of the microscope 

 to the study of these deposits brings us in contact with the greatest 

 <|tiestioiis not only of terrestrial, but also of cosmical physics, and 

 furnishes evidence of the highest value for their solution. 



1 See Sir A. Geikie on ' Geographical Evolution,' Proc. Soy, Geog. Soc. July 1879 ; 

 and for detailed results ' Preliminary Report of Cruise of " Challenger " ' (Wyville 

 Thomson), Proc. Roy. Soc. vol. xxiv. (1876) p. 463, and ' Challenger ' Reports (Murray 

 and Renard), Deep Sea Deposits, p. 327. 



