THE EARTH AS A WHOLE 3 



meters. In the southern Pacific the Tonga deep has an area of 

 123,000 square kilometers below 7,000 meters, and 63,000 square 

 kilometers below 8,000 meters, while the Kermadec deep to the 

 south of it has 137,000 square kilometers below 7,000 meters, and 

 78,000 square kilometers below 8,000 meters. The maximum 

 depths of these two depressions are 9,427 and 9,413 meters, re- 

 spectively. (Kriimmel-17, i :^5.) The total area of the oceanic de- 

 pressions below 6,000 meters is given by Penck as 10.6 million 

 square kilometers, and by Kriimmel as 5.4 million square kilo- 

 meters. Taking the area of the seas as a whole at 361,100,000 

 square kilometers, and evening out the irregularities of the bot- 

 tom, so as to obtain a mean depth, we find this to be approximately 

 3,680 meters or 2,012 fathoms (see further under lithosphere). 

 X'arious estimates as to the volume of the sea water as a whole 

 have been made. The most recent of these, given by Kriimmel, 

 places it at 1,330 million cubic kilometers. The weight of the sea 

 water may be calculated from this by taking its average density, 

 increased in the deeper portions by pressure, as 1.04, the result 

 being 1.3832 X 10^® metric tons or in round numbers 138 X 10 ^° 

 metric tons.* 



Transgressions and Regressions. Two types of alternation of 

 the level of the hydrosphere are recognized : the local and the uni- 

 versal. The former is due to slight up or down warpings or fault- 

 ings of parts of a continent with resulting local retreat or advance 

 of the sea. The effects of such a change will be noticed only locally 

 and will at best be of slight amount. An example of this is shown 

 in the periodic rising and sinking of the land in the Gulf of Naples 

 as recorded in the ruins of the temple of Jupiter Serapis, near 

 Pozzuoli (Lyell-i8, ii Chapter XXX) ; or, in the Scandinavian 

 region, where, during the last century, an area of 300,000 square 

 kilometers rose on the average 0.7 meter, making a total elevated 

 mass of 210 cubic kilometers. 



The second type of change is one that afifects the entire surface 

 of the ocean, and is best explained by a bulging up or a sinking down 

 of the sea bottom, the result of which will be a universal rise of 

 the sea-level and transgression over, or fall and retreat from the 

 edge of the land. For these latter movements Suess (27, ii:d5*o; 

 28, ii:5j^) has proposed the term: "Eustatic Movements." The 

 sinking of the sea bottom and consequent lowering of the level and 

 withdrawal of the water from the land constitute negative eustatic 

 movements, while elevation or bowing up of the sea bottom with 



* The metric ton contains 1,000 kilograms, equivalent to 2,204.6 pounds 

 avoirdupois. 



