iS95- MURRAY'S MUD LINE. 397 



100 fathoms to be about the average depth at which it is formed on 

 continental shores facing the great ocean basins. Alexander Agassiz, 

 one of the greatest living authorities on deep-sea dredging and marine 

 zoology, in a review of the Summary volumes of the " Challenger " 

 report, says with reference to the position of the mud-line : — 



" Undoubtedly the loo-fathom line is an important limit of depth 

 and indicates usually the edge of the oceanic continental slope. Yet 

 it seems to us as if the more limited deep-sea explorations of the Blake 

 and the Albatross along continental slopes indicated that in many 

 localities there was considerable variety in the nature of the bottom, 

 as, for instance, in volcanic and coral-reef districts and off rocky shores, 

 where faunal conditions of very great variety extend to depths 

 considerably beyond the mud-line and approach very near the 

 500-fathom line. We should be inclined to extend the limit of uni- 

 formity into greater depths than 100 fathoms along the oceanic 

 continental slopes — certainly to 150, and often to more than 300 

 fathoms. 



" While we fully agree with Murray in considering the fauna 

 living near the loo-fathom line as probably that from which the deep- 

 sea faunae were derived, it seems somewhat hazardous, in our present 

 state of knowledge, to affirm as emphatically as he does that the 

 majority of the animals living within the loo-fathom line have pelagic 

 larvae, while the majority of those living at the mud-line have a direct 

 development like those found in very deep water." 



Thus, while Professor Herdman appears to argue that the mud- 

 line should be placed at some depth less than 100 fathoms, 

 Professor Agassiz thinks that it should be placed at a greater depth. 

 But, making allowance for local conditions and variations. Dr. Murray 

 is probably near the truth in assigning an average depth of about 100 

 fathoms to the mud-line off continental shores bordering the great 

 oceans. Dr. Murray says that in and on the thin red watery surface- 

 layer of a blue mud there are many living organisms, while in the 

 stiff blue layers beneath organisms are much less abundant. 



James Chumley. 

 " Challenger " Office, Edinburgh. 



