MARINE DEPOSITS 217 



deep waters of tropical regions than in the 

 warm surface waters, the calcareous structures 

 becoming less massive with increasing depth. 

 Although lime-secreting organisms are so 

 abundant in tropical surface waters, their 

 shells and skeletons are rare or entirely 

 absent from large areas of the ocean-floor 

 in the greatest depths. Pteropod ooze for 

 instance is limited to the comparatively 

 shallow depths of the warmer oceans, 

 and yet it has been shown by hundreds 

 of observations that pteropods and hetero- 

 pods are as abundant at the surface over 

 areas where not a trace of their shells can be 

 detected in the bottom-deposits as over areas 

 where they have accumulated to such an 

 extent as to form a pteropod ooze. The same 

 holds good with reference to the shells of 

 pelagic foraminifera and calcareous pelagic 

 algae. When a series of deposits is examined 

 from the same tropical area, but from different 

 depths, it is found that in depths of between 

 say 500 and 1000 fathoms nearly every species 

 of calcareous shell taken by the tow-nets in 

 the surface waters may be observed in the 

 deposit at the bottom. In greater depths — 

 say between 1000 and 2000 fathoms — all 

 the thinner and more delicate shells have 

 disappeared from the deposit, especially the 

 pteropod, heteropod, and the smaller and more 



