CONDITION OF THE FLOOR OF THE OCEAN. 313 



lenger, Tiiscarora, and Supau deeps; and in the South Pacific within 

 the Aldrich and Richards deeps. Depths exceeding .5,000 fathoms 

 have been hitherto recorded only within the Aldrich Deep of the South 

 Pacific, to the east of the Kermadecs and Friendly Islands, where the 

 greatest depth is 5,155 fathoms, or 530 feet more than 5 geograph- 

 ical miles, being about 2,000 feet more below the level of the sea than 

 the summit of Mount Everest in the Himalayas is above it. The 

 levels on the surface of the lithosphere thus oscillate between the 

 limits of about 10 geographical miles (more than 18 kilometers). 



TEMPERATURE OF THE OCEAN FLOOR. 



Our knowledge of the temperature on the floor of the ocean is 

 derived from observations in the laj^ers of water immediately above 

 the bottom by means of deep-sea thermometers, from the electric 

 resistance of telegraph cables resting on the bed of the great ocean 

 basins, and from the temperature of large masses of mud and ooze 

 brought up by the dredge from great depths. These observations are 

 now suflicienth^ numerous to permit of some general statements as to 

 the distribution of temperature over the bottom of the great oceans. 



All the temperatures recorded up to the present time in the subsur- 

 face waters of the open ocean indicate that at a depth of about 100 

 fathoms seasonal variation of temperature disappears. Bej^ond that 

 depth there is a constant, or nearly constant, temperature at any one 

 place throughout the year. In some special positions, and under some 

 peculiar conditions, a lateral shifting of large bodies of water takes 

 place on the floor of the ocean at depths greater than 100 fathoms. 

 This phenomenon has been well illustrated by Professor Libbey off' the 

 east coast of North America, where the Gulf Stream and Labrador 

 Current run side by side in opposite directions. This lateral shifting 

 can not, however, be called seasonal, for it appears to be effected b}^ 

 violent storms or strong offshore winds bringing up colder water from 

 considerable depths to supply the place of the surface drift, so that 

 the colder water covers stretches of the ocean's bed which under nor- 

 mal conditions are overlaid by warmer strata of water. Sudden 

 changes of temperature like these cause the destruction of innumerable 

 marine animals, and produce very marked peculiarities in the deposits 

 over the areas thus affected. 



It is estimated that 92 per cent of the entire sea floor has a tempera- 

 ture lower than 40° F. This is in striking contrast to the temperature 

 prevailing at the surface of the ocean, only 16 per cent of which has 

 a mean temperature under 40° F. The temperature over nearly the 

 whole of the floor of the Indian Ocean in deep water is under 35° F. 

 A similar temperature occurs over a large part of the South Atlantic 

 and certain parts of the Pacific, but at the bottom of the North Atlan- 

 tic basin and over a very large portion of the Pacific the temperature 



