18 THE OCEAN RIVER 



tides or electrons, and gamma rays, which are very penetrating 

 short-wave X-rays. The rate at which the various particles are 

 given off has been measured in the laboratory. It is known 

 from this that any mass of pure uranium would, in the course 

 of 4,500 million years, give off radiation sufficient to convert 

 half of it into lead, helium and intermediate products. Before 

 the molten rocks of the earth became crystallized, the helium 

 produced by the steady degradation of the uranium was lost, 

 but after the Earth's crust became solid the slowly accumulated 

 helium and lead remained trapped in the rock. The proportion 

 of helium or lead to unchanged uranium, therefore, when 

 measured under proper conditions, is an index of the age of 

 rocks from the time they congealed. In this way it has been 

 found that the most ancient rocks known on earth are over 

 1,800 million years old; and it is probable that there are still 

 older, inaccessible rocks buried beyond our reach. 



Somewhat later, when the earth had cooled sufficiently, the 

 torrential acid rains began to pour incessantly down upon the 

 hot rocks, in an atmosphere lacking oxygen and life of any 

 form, and in the Stygian gloom of an enormous cloud blanket 

 which shut off the sun's rays. At some still more recent time, 

 the rocks must have cooled sufficiently for the rain, instead of 

 evaporating instantly, to remain upon the earth, and to roll 

 down from the bare hills and collect in the depressions and 

 basins. It is not unlikely that one of these basins was the first 

 bed of the Ocean River — though vastly different in shape 

 from the North Atlantic as we know it today. 



Some of the clues from which we deduce the manner in 

 which the Atlantic basin was formed come from the modern 

 science of seismology. When an explosive charge is set off in a 

 deep well or at the bottom of the ocean, or when an earth- 

 quake occurs, the geophysicist is able to measure with his 

 instruments the speed at which the vibrations travel through 

 the rocks far below his feet. The speed and nature of these 



