424 THE CANADIAN NATURALIST. [Vol. vii. 



that of bottom-water in the tropics is nearly the same as in the 

 pohir area. Why then does the bottom-water of the tropics, 

 being of lower salinity, underlie the more saline strata ? Because 

 the density it lacks from its lower salinity is more than compens- 

 ated by the lowness of its temperature. Passing, however, from 

 either tropic towards the Equator, the salinity of surface-water 

 is found to diminish, until its specific gravity is reduced from 

 1027-3 to 1026-4 or 1026-3, which is that of the polar under-flow. 

 Lenz adduced the low salinity of the surface-water under the 

 Equator as evidence of the rise of polar water from the bottom, 

 and showed that there is a band of water at the Equator colder 

 than any to the north or south of it. 



The Oceanic Circulation thus produced brings every drop of 

 water in turn to the surface, enabling it to part with carbonic 

 acid and to absorb oxygen ; this, then, is its importance to Ani- 

 mal Life. From the analysis of gases dissolved in the water of the 

 oceanic area, it was found that, for 45 per cent, of carbonic acid 

 there was usually from 16 to 20 per cent, of oxygen — this being 

 the result of a series of observations taken off Ireland and Scot- 

 land at various depths down to 2000 fathoms. This amount 

 of oxygen is sufiicient to support a large quantity of Animal 

 Life, in spite of the, to air-breathers, fatal proportion of carbonic 

 acid — if indeed the carbonic acid be not in a liquefied, and thus 

 perhaps more innocuous form. 



In the Mediterranean totally different conditions prevail. It 

 was expected that a Tertiary fauna would be found at great 

 depths, analogous to the Cretaceous-like fauna of the ocean out- 

 side. Instead of that, only a viscid mud, almost devoid of life, 

 was brought up. The western basin has a depth of 1600 fathoms, 

 the eastern basin one of 2000 fathoms; the bottom temperature 

 is nearly uniform at about 55° F., a great difference in thermal 

 condition from the Atlmtic. The reason is that the Mediter- 

 ranean is cut off entirely from the polar uuder-fiow, which, off 

 Lisbon, produces a temperature of 40° F. at a depth of 700 

 fathoms, and 36 J° at 1500 fathoms. In the Mediterranean, on 

 the other hand, we have a surface temperature from 60° to 70° 

 F., which, in the first 100 fathoms, falls to 54° or 55° F., below 

 which to the bottom, no matter at what depth, there is no change 

 at all, but a slight variation according to latitude, due in part 

 to the mean winter temperature of the localit}-. The whole of 

 the lower portion, therefore, below the influence of the Gibraltar 



