668 DYNAMICAL GEOLOGY. 



per cent; and in superficial clays and gravels it is at least 10 per 

 cent. 



If the thickness of the supercrust over the continental portion of 

 the gjobe averages five miles, and the average volume of moisture in 

 the formations, both metamorphic and unaltered, be 2*5 per cent., the 

 whole amount of water absorbed and confined would be a fortieth of 

 five miles, or about 650 feet in depth, for the area of the continents. 

 The deposits over the oceanic basins have relatively little thickness. 

 Whatever reasonable allowance be made for them, the whole loss to 

 the ocean waters, in depth, from this source, will not exceed 400 feet. 

 This confined water, while a feeble agent of change at the ordinary 

 temperature, is one of immense importance when much heat is present. 



As Delesse states, the water confined in terrestrial plants and animals is another 

 part taken permanently from the oceans, since the commencement of Paleozoic time. 



2. THE OCEAN. 

 1. Oceanic Forces. 



The ocean exerts mechanical force, by means of its — 

 1. General system of currents; 2. Wind-waves and currents; 3. 

 Tidal waves and currents ; 4. Earthquake-waves. 



The force of moving salt water is the same as for fresh water, ex- 

 cept the difference arising from the greater density of the former, — 

 its specific gravity being one thirty-fifth to one-fortieth more than that 

 of fresh water, and a cubic foot weighing 64 lbs. 



The specific gravity of sea-water varies for different parts of the ocean. For the 

 waters of the Southern ocean, it is 1*02919 ; the Northern, 1*02757 ; equatorial 1*02777; 

 Mediterranean Sea, 1*0270-1*0294; Black Sea, 1*01118 (Marcet). In most seas receiv- 

 ing large rivers, and in bays, the density is least. The specific gravity of the water of 

 East River, off New York City, at high tide, is L020.J8 (Beck). 



1. General System of Currents. 



The system of oceanic currents is briefly explained on pages 38- 

 42. It is part of the organic structure of the globe, irrespective of 

 its age or condition ; for, whatever the temperature of the poles, there 

 must always have been warmer tropics, under the path of the sun. 



The prominent characteristics of these currents, bearing on their 

 mechanical effects in geological history, are the following : — 



1. The rate of movement is slow. — The maximum velocity of the 

 Gulf Stream is five miles an hour, and the average less than one 

 mile and a half. 



The Gulf Stream is most rapid off Florida, where the hourly rate is three to five 

 miles; off Sandy Hook, it is one mile and a half. The rate of flow of the polar current 



