686 OCEANOGRAPHY 



the tile-fish was nearly exterminated off the United States coasts 

 in 1882, it was estimated that over hundreds of square miles there 

 was a layer of dead marine fishes and other animals on the bottom 

 six feet in depth. This vast destruction of marine organisms points 

 clearly to the source of the phosphate in these deposits, and we 

 obtain a hint as to the conditions under which greensand and sim- 

 ilar rocks were laid down in past ages. Generally it may be said that 

 in the terrigenous deposits along continental shores we have rocks 

 now in process of formation which resemble closely the stratified 

 rocks of the continents, so that these rocks may all be said to 

 have been formed in varying depths in inclosed seas or along the 

 continental slopes within two or three hundred miles from land. 



It is quite different when we turn to the marine deposits now in 

 process of formation towards the central portions of the great ocean 

 basins. No geologist has yet been able to produce a specimen of a 

 stratified rock which can with certainty be said to have been built up 

 under conditions similar to those under which the typical red and 

 chocolate clays, the Pteropod and Globigerina oozes, the Radiolarian 

 and Diatom oozes of the central oceanic regions are laid down at the 

 present time. These pelagic deposits cover considerably more than 

 one half of the surface of our planet. The typical pelagic deposits are 

 principally made up of the shells and skeletons of calcareous and 

 siliceous organisms now living in the surface waters and of inorganic 

 material derived from submarine eruptions, or of pumice and volcanic 

 dusts floated or wind-borne from volcanic areas. The calcareous or- 

 ganisms play a most important r61e in the pelagic deposits, and their 

 greater or less abundance, or complete absence, is more or less puz- 

 zling to the oceanographer. If, for instance, we should find in the trop- 

 ical or subtropical regions of the ocean a cup-shaped or horseshoe- 

 shaped elevation rising from the deep floor of the ocean, having a dia- 

 meter, say, of fifty miles across, and the summit or edges of the cup 

 rising to within 6000 feet of the surface of the ocean, while in the interior 

 and on the outside of the cup the bottom descended to 20,000 feet be- 

 low the waves, then we should find on the elevated edges of the cup 

 deposits made up of 90 per cent of calcium carbonate, consisting almost 

 wholly of the remains of pelagic organisms. As we descend into the 

 hollow of the cup, or into the depths outside the cup, these organic 

 remains would slowly disappear, till in the deposit at the bottom in 

 20,000 feet hardly a trace of calcareous organisms would be found, 

 and the deposits there would consist of a red or chocolate clay derived 

 from volcanic ejecta,with manganese-iron nodules, earbones of whales, 

 sharks' teeth, and some cosmic spherules derived from meteorites. 

 This hypothetical case represents what is* found again and again 

 throughout the ocean basins. Where exactly similar surface condi- 

 tions prevail at the surface of the ocean two wholly different marine 



