134 T. C. CHAMBERLIN 



from all migratory connections with land-girting bottoms of like 

 depth, may come to bear a pure pelagic deposit free from clear 

 evidences of shallow-water bottom life and yet without necessarily 

 implying any great depth. 



(2) The relics of the life of the zone that lies below the super- 

 ficial and photosynthetic zone and above the bottom. Normally 

 the life relics of this median zone, as well as the relics from the 

 surface zone above, fall together to the bottom and are there mingled 

 with the relics of organisms that live at the bottom. Only a part 

 of the mixture is diagnostic of depth, the benthos. A possible 

 source of error of interpretation may arise in those areas in which 

 the lower waters are constantly welling up and so displace the usual 

 surface layer and more or less of its life. This displacement of 

 the surface life is most likely to be effective where the temperature 

 or the salinity of the rising waters is uncongenial to the surface life. 

 In such areas the lower life is likely to follow the rising water to 

 unusual heights and perhaps thus to vitiate more or less the usual 

 bathymetric interpretations. 



(3) The relics of the bottom life. In so far as this life is strictly 

 confined to the bottom and can be proven to be limited to given 

 depths, it constitutes a firm criterion for determining the depth 

 at which the deposits containing it were formed. Positive proof 

 that any particular form of life is strictly confined to given great 

 depths is attended by inherent difficulties. In the great depths of 

 the ocean basins there is a complication of the influences that afi^ect 

 living organisms, (i) pressure, (2) temperature, (3) salinity, (4) gas- 

 content, as well as less tangible agencies, and it is improbable that 

 the individual effects of these have as yet been wholly disentangled 

 and the influence of pressure, as such, separately discriminated. 

 Pressure ^ however, is the only true criterion of depth; the associated 

 temperatures, salinities, and gas-contents are incidental; indeed 

 just now they are probably the special results of the present 

 polar phase of the deep-sea circulation; they are perhaps to be 

 regarded as but a lingering feature of the recent glacial period, and 

 as more or less inapplicable to other periods. It would probably 

 be quite unsafe to assume similar temperatures, salinities, and gas- 

 contents at all other times. In the period, for example, during 



