264 SCHUCHERT— BLACK SHALE DEPOSITION. [May 7, 



studied in Kriimniel's " Handbuch der Ozeanographie," I., 1907, 

 pages 292-317. Furthermore, the amount of oxygen is increased 

 when there is an abundance of assimilating plants, as in the areas 

 of the sea-weeds and diatoms. The gases are then distributed by 

 the general water circulation to most parts of the oceans and even 

 into the greatest depths. In general, there is an abundance of 

 oxygen down to 350 feet, but in the tropics it is wanting in the 

 greater depths of the shelf seas. The oxygen is consumed by the 

 animals and by various hydro-chemical processes and consequently 

 diminishes in quantity as it is carried down from the surface and 

 over the bottom, but the quantity of nitrogen remains constant. Sir 

 John Murray states further that in the streaming open ocean of 

 today there is usually an abundance of oxygen even at the greatest 

 depth, due to the sinking heavier and colder polar waters, but this 

 is not the case in partially enclosed seas which are more or less cut 

 off by barriers and where the water is said to be " stale," and in the 

 deeper layers of which vertical circulation is restricted. 



Similar stagnant conditions "prevail in several Norwegian 

 'threshold fjords,' or on a smaller scale in the oyster-' polls.' In 

 such places the bottom is thickly covered with organic matter; a 

 slimy black mud is formed, swarming with bacteria that produce 

 sulphuretted hydrogen, which spreads through the water, combin- 

 ing with the oxygen to form various sulphates. This causes the oxy- 

 gen to decrease and finally to disappear altogether, when the sulphur- 

 etted hydrogen begins to appear free in solution. It gradually spreads 

 upwards, until the water is devoid of oxygen and contains free sul- 

 phuretted hydrogen, at a depth of only 100 fathoms in the Black 

 Sea, and in the oyster-basins in autumn often at merely a couple of 

 meters below the surface. In summer the ' bottom-water ' of the 

 oyster-' polls ' lies stagnant, but in the course of the autumn and 

 winter it is generally renewed by the supply of comparatively heavy 

 water from without; then the sulphuretted hydrogen disappears 

 and the oxygen returns, producing thus an annual change in the 

 gaseous conditions of the deeper parts of the oyster-' polls.' In 

 autumn the state of things may become critical for the oysters, 

 which are suspended in baskets at a depth of 1^-2 meters; it hap- 



