i68 DEPTHS OF THE OCEAN 



in much deeper water than in any other of the great ocean 

 basins. 



Diatom ooze occurs typically only in extra-tropical regions, 

 forming a broad almost circumpolar band in the great Southern 

 Ocean, outside the zone of Blue mud bordering the Antarctic 

 continent, and a smaller band along the extreme northern border 

 of the Pacific Ocean, along the Alaskan and British Columbian 

 coasts of North America, and the Kamtchatkan and Japanese 

 coasts of Asia and the intervening Aleutian Islands. 



Radiolarian ooze covers the sea-floor in certain portions of 

 the tropical regions of the Pacific and Indian Oceans, being 

 apparently entirely unrepresented in the Atlantic ; it occurs in 

 a band of varying width in the equatorial eastern Pacific, 

 approaching comparatively close to the shores of Central 

 America, and in other smaller isolated areas. 



Red clay is the most characteristic and most extensive of 

 the pelagic deposits, occupying the deepest portions of the great 

 ocean basins except in the polar regions, extending beyond 

 lat. 50'' N. and S. in the Pacific, and between lat. 40° N. and S. 

 in the Atlantic. It is the typical deposit of the great Pacific 

 Ocean, attaining there its maximum development, and being 

 associated over wide areas with the characteristic manganese 

 nodules; in the Indian Ocean it is also associated with much 

 manganese, and therefore usually of a dark chocolate colour, 

 while in the Atlantic it is generally intermixed with less 

 manganese and usually of a light red-brown colour. 



As regards the vertical distribution of the deposits, we have 

 already indicated how gradual is the transition between the 

 various types and classes, so that frequently two or more names 

 might be used to characterise samples from the border regions. 

 It is therefore evident that no definite limits of depth can be 

 assigned to the different types of deposits, but their general 

 distribution m.ay be broadly outlined. 



The terrigenous deposits have for their upper limit the 

 shore-line, while their lower limit varies according to local con- 

 ditions. We have already pointed out that in certain localities 

 Blue mud may be restricted to the continental slope within 

 depths less than 1000 fathoms, while in other localities it may 

 extend far into the abysmal area in depths exceeding 2000 

 fathoms, and in some places approaching 3000 fathoms. Coral 

 mud may extend into depths approaching 2000 fathoms before 

 passing gradually into a Globigerina ooze, but sometimes it 

 merges into Pteropod ooze in depths less than 1000 fathoms. 



