MURRAY — DEPTH AND MARINE DEPOSITS. 371 



General Conclusions. 



An examination of the foregoing paragraphs and the accompanying maps with refer- 

 ence to the distribution of marine deposits over the floor of the Indian Ocean at all 

 depths leads to some very interesting conclusions. It may be stated generally that 

 organisms secrete calcium carbonate much more abundantly and rapidly in warm than 

 in cold water. In the Arctic and Antarctic Oceans and in the deep sea, where tempe- 

 ratures approaching the freezing-point of fresh water prevail, there are no great accumu- 

 lations of calcium cai'bonate due to secretion by benthonic organisms. The calcareous 

 shells and skeletons secreted in these cold waters are thin and slender when compared 

 with similar structures found in tropical seas. 



In tropical and subtropical oceans, where the temperature exceeds 65° F., and where 

 there is an annual range of temperature not exceeding 10° or 15° F., there is the most 

 abundant secretion of calcium carbonate, both by benthonic and planktonic organisms. 

 The coral-reefs and the Pteropod and Globigerina Oozes are striking examples of this 

 statement. The coral-reefs are limited in their distribution by the temperature con- 

 ditions above mentioned. It is not possible to state so definitely the temperature 

 conditions which limit the distribution of shelled Pteropods, Heteropods, pelagic 

 Foraminifera, and calcareous Algse ( Coccospheres and Hhabdospheres), but it may be 

 stated generally that while very many species of these organisms are found everywhere 

 in the warm and Salter waters of the tropics, only one or two species extend towards the 

 cold waters of the Polar oceans, or into the less saline waters found around continental 

 shores and in partially enclosed seas. 



It is probably well within the mark to say that over 90 per cent, of the calcium 

 carbonate -found in the Pteropod Oozes, Globigerina Oozes, Diatom Oozes, Radiolarian 

 Oozes, and Red Clays is derived from the shells and skeletons of Ume-secreting organisms 

 which lived in the surface waters of the ocean. The calcareous shells and skeletons 

 derived from benthonic organisms may be said to be relatively rare in these deeper 

 marine deposits. 



It has been pointed out that the Pteropod Oozes are limited to the comparatively 

 shallow depths of the ocean. When these observations were first recorded it was 

 argued that Pteropods and Heteropods were more abundant in the surface waters over 

 these places where they were found at the bottom, or that these shells had been accumu- 

 lated at these spots by submarine currents. While it may be admitted that these 

 molluscs are more abundant in some currents, where there are waters of higher tempe- 

 rature and salinity, than at other places, still it has been shown by hundreds of obser- 

 vations that they are as abundant at the surface over areas where not a trace of their 

 shells can be found in the deposits as over areas where they have accumulated to such 

 an extent as to give the chief characteristic to the deposit called Pteropod Ooze. It 

 follows that the shells must have been removed by solution while falling through the 

 water to the greater depth or shortly after they reached the bottom. 



What is here stated with reference to the Pteropod and Heteropod shells holds also 

 for the shells of pelagic Foraminifera and calcareous pelagic Algae. When a series of 



SECOND SERIES.— ZOOLOGY, VOL. XIII. 60 



