198 PHYSICAL GEOLOGY 



aids marine organisms in the formation of their calcareous shells and 

 skeletons, and since this compound is most abundant in warm waters, 

 it is probable that when the shells of fossils are thick, the water in 

 which they lived was warm. Thus the existence of thick-shelled, 

 Paleozoic fossils in the rocks of the Arctic region indicates that when 

 they were alive, the waters in that region were much warmer than 

 now. It is evident from the above that in order to understand the 

 life and physical conditions of the remote past a knowledge of the 

 habits and conditions of life of animals now living is necessary. 



Age of the Ocean. — Attempts have been made to estimate the age of the ocean 

 from the quantity of salt dissolved in it. Such estimates are based on the as- 

 sumption that all of the salt of the ocean has been derived from the weathering and 

 erosion of rocks and has been carried to the seas by streams. The simplest form of 

 the problem assumes that the age of the ocean may be determined by dividing the 

 total amount of salt in it by the amount of this mineral carried to the sea each year 

 by streams. The amount of salt in the ocean can be determined with considerable 

 accuracy, since the composition of sea water varies little in different parts of the world, 

 and the approximate total volume of the ocean is known. There are, however, a 

 number of doubtful elements in the problem, (i) The amount of salt discharged by 

 rivers may have varied from time to time. The rate of discharge has undoubtedly 

 been hastened through human agency. The importance of this factor is seen in the 

 fact that 14,500,000 metric tons of common salt are mined or extracted from salt 

 wells yearly. If thi^ is annually returned to the ocean, it is evident that the present 

 rate of discharge is higher than in the past. (2) The salt blown upon the land from 

 the ocean is considerable and must be deducted from the total carried in. (3) The 

 salt received by the decomposition of the rocks by marine erosion (p. 202), and from 

 volcanic ejectamenta must be subtracted. (4) Much salt once in the ocean is now 

 stored within the sedimentary rocks. 



When the known factors are considered, it is " inferred that the age, of the ocean, 

 since the earth assumed its present form, is somewhat less than 100,000,000 years." x 



The amount of calcium carbonate in the oceans cannot be used as a basis for an 

 estimate of their age, since some of it is precipitated upon reaching the salt water, 

 and much of it is used by animals and plants for their skeletons and shells. 



Movement of the Water 



Wave Motion. — Since marine erosion is accomplished chiefly by 

 wave action, it is important to know something of the theory of wave 

 motion, of the height and force of waves, and of the depth to which 

 they are effective. Storm waves are set in motion as a result of the 

 friction between the wind and water. The water appears to move 

 forward, just as do the waves in a field of grain which is agitated by 

 the wind. If a pebble is thrown into a pond on a calm day, waves 

 1 Clarke, F. W., — Bull. U. S. Geol. Surv. No. 490, p. 142. 



