172 PRINCIPLES OF STRATIGRAPHY 



The amount of dissolved gases depends directly on the pressure 

 on the water. Thus a lake lying in lowlands contains more dis- 

 solved gases than a mountain lake, where the pressure is less. With 

 rising barometer and sinking temperature the upper strata of water 

 of a lake will absorb more gases, while with falling pressure and 

 rising temperature gases will be given off. Change in temperature 

 is, however, more important than change in pressure. With a rise 

 from o° to 25° C. the power of water for absorption of gases de- 

 creases from 30 to 40%, while the extremes of pressure bring about 

 a change in absorption of only about 6%. The surface waters of 

 lakes are generally saturated with atmospheric gases, though the 

 presence of living animals and plants and the decay of organic mat- 

 ter tend to disturb the normal balance. In the deeper strata of the 

 waters of lakes the power to absorb gases is much greater, owing 

 to the increased pressure, but as a matter of fact the actual amounts 

 correspond more nearly to those obtained in surface waters during 

 periods of low temperature. This is due to the fact that the deeper 

 waters depend for their atmospheric gases upon the amount fur- 

 nished them by the upper layers, when supersaturated, this amount 

 being greatest in the cold period ; or upon the amount carried down 

 to the deeper strata by surface waters acting as convection cur- 

 rents, when the temperature of the surface has sunk to that of 

 the deeper strata. In the deeper strata of lake waters a deficit in 

 oxygen and an increase in CO, may further develop through the 

 physiological activities of bottom organisms or the decay of organic 

 matter. This change is less marked in surface waters, owing to the 

 balancing effect of chlorophyll-bearing plants, which are absent in 

 the deeper waters. (See Chapter XI.) 



From the tables above given it is clear that the proportion of 

 oxygen to nitrogen is much higher in the absorbed air of water 

 than in the normal atmosphere. The latter has 21 parts of oxygen 

 to 78 parts of nitrogen, the proportion being in round numbers as 

 I : 4. In the absorbed air of pure fresh water the proportion of O 

 to N at 0° is as 34.6:61.8, and at 30° as 33.3:63.6, or, in round 

 numbers, in either case as 1:2. In applying this fact, however, to 

 the available oxygen for aqueous plant and animal life, it must be 

 borne in mind that, though the proportion of oxygen to nitrogen 

 is greater in water than in air, the absolute amount of oxygen per 

 liter of the medium in which the organism breathes is vastly greater 

 in air than in water, being 210 c. c. lor each liter of air, and only 

 10 c. c. for each liter of water. 



Organic Matter. This is present in solution in many waters, 

 especially those of rivers and lakes. Calculated in percentages of 



