5 6 EVOLUTION OF THE EARTH 



thinks, came forth out of the earth itself, through volcanic 

 activity and thermal springs. Even though some of the sur- 

 ficial waters vanish through soaking into the cold earth-shell, 

 their volume has increased throughout geologic time; the 

 greatest amount was added during the later part of the long 

 growing period of the earth and during the Archeozoic era, 

 when from 25 to 50 per cent of the present volume is believed 

 to have come into existence. The rest has been added during 

 subsequent geologic time. 



Source of the salts of the oceans. It is well known that the 

 seas and oceans are salty, and that on the average there are 

 3.5 pounds of saline matter to every 100 pounds of marine 

 water, and in each 100 pounds of sea salts there are nearly 78 

 pounds of sodium chloride or our table salt. Clarke has esti- 

 mated that if all of the saline matter of the oceans were con- 

 centrated, and the volume placed on the United States, its 

 surface would be completely covered to a depth of 1.6 miles. 

 As all of this saline matter has been leached out of the rocks 

 of the dry lands since the earth has had rains, and as very little 

 of it, comparatively, has been taken out of the ocean by the 

 accumulating rocks, it has been further estimated that it repre- 

 sents the breaking down of a mass of average igneous rock 

 equal to at least 6,900 feet in thickness over all the continental 

 platforms. Probably it is more correct to state that the con- 

 tinents have suffered erosion of igneous rocks amounting to 

 between I and 2 miles of average depth. Of course all 

 erosion throughout geologic time was far greater, perhaps, 

 as Barrell states, from 50 to even 100 per cent higher. It 

 included the reworking of older materials, igneous and sedi- 

 mentary. Furthermore, "more than a half, perhaps four- 

 fifths, of the erosion of igneous rocks was accomplished before 

 the beginning of the Paleozoic" (Barrell). 



Various computations have been made as to how long it 

 has taken the salts in the oceans to accumulate and the best 



