1046 PRINCIPLES OF STRATIGRAPHY 



of the bottom layers is greater than that of the surface, vertical 

 currents are slight, and hence the lower strata are poor in oxygen 

 and unable to support animal life. Thus in the Black Sea animal 

 life is practically absent below a depth of 125 fathoms, though 

 dead plankton ic organisms sink to the bottom, where they are 

 decomposed by anaerobic bacteria. The great depths of the Black 

 Sea are covered by a layer of black mud. from which H^S is 

 abundantly separated and which is rich in pelagic diatoms and frag- 

 ments of very young pelecypods from the meroplankton. The 

 presence of abundant PLS in these depths is characteristic. It is 

 separated by the sulphobacteria from the dead organic matter and 

 the sulphates of the sea water. Accompanying it is the formation 

 of iron sulphide and the separation of carbonates, especially CaCO,. 

 Pure atmospheric air contains almost 21 per cent, of oxygen and 

 about 79 per cent, of nitrogen (see ante. Chapter II). There is 

 always, however, some carbon-dioxide and water vapor present, 

 the quantity varying with the temperature and the movements of 

 the air, besides being very variable in different localities. 



Carbon-dioxide, while necessary to plant life, is injurious to 

 the higher types of animals, if the proportion is above i to 2,000 

 of volume. A somewhat larger quantity, however, has no serious 

 effect on some lower forms of terrestrial animals, and in some 

 cases it may even be beneficial to them. The moisture in the 

 air is necessary to the existence of most terrestrial organisms, many, 

 indeed, being unable to exist in a region where the percentage of 

 water vapor in the air is low. Nevertheless, some animals are 

 found in extremely dry regions, and these are not infrequently types 

 which belong normally to moist climates. Land snails, for ex- 

 ample, generally require much moisture, and large numbers of 

 such animals perish during the dry summers, or survive only by 

 burying themselves in the earth or in crevices of rocks, and closing 

 the mouth of the shell with a membranous operculum. In deserts 

 these snails can obtain moisture only during the night or early 

 morning from the heavy dews which result through excessive radia- 

 tion, and it is only during these times that they lead an active life. 



In the saturated atmosphere of the tropical forests are found 

 many types of animals which are normally aquatic. Thus a type 

 of leach, allied to the medicinal leach of the fresh water ponds, 

 lives on trees in the moist forests of India and the Indian islands; 

 and a number of species of land planarians are known, mostly from 

 the tropics. Many amphipod Crustacea of the family Orchestidse, 

 or beach fleas, live exclusively on land, though they have the gills 

 of the true water species. These require a very moist atmosphere. 



