FIRE, ROCK, AND SEA 19 



vibrations give definite indications of the type of rock at dif- 

 ferent depths below the surface. Another tool of the scientist 

 who seeks to learn the structure and life history of the Ocean 

 River basin is nothing more than a glorified pendulum. One 

 of the useful properties of a pendulum is that it swings at a 

 faster rate when the pull of gravity increases. Thus, at points 

 of the earth's surface below which there is more than the aver- 

 age amount of matter, or the rocks are heavier than elsewhere, 

 the pendulum swings will slightly speed up. Where the rocks 

 are lighter or for some reason there is a deficiency of matter, 

 the pendulum slows down. 



Using these delicate measuring instruments, the geophysi- 

 cist has discovered that the earth consists of three principal 

 layers. The heavy central portion consists of almost pure iron 

 and nickel, at such high temperatures and pressures that it is 

 completely molten. This extends only about halfway to the 

 surface. Above it is a 700-mile-thick layer of very hot semi- 

 liquid heavy rock of the type known as basalt, about one-third 

 the weight of the iron core, or about four times the density of 

 water. It forms a plastic layer which is more liquid toward the 

 inside and becomes more and more viscous until the outer 

 solid crust is reached. Above the plastic basalt and its thin 

 solid outer layer is an upper crust of solid granite about 30 

 miles thick, lighter than the underlying basalt, with a density 

 less than three times that of water. 



According to one theory the layer of granite is believed to 

 have formed a thin but complete crust around the earth at the 

 time the moon was formed. A striking confirmation of this 

 theory of the moon's origin comes from our knowledge of the 

 earth's structure, and from astronomical calculations that 

 show the moon to be made up mainly of materials similar in 

 density to the granite and basalt of the earth's crust. The moon 

 is believed by some to have left a great gap in the earth's sur- 

 face as the huge mass of granite and basalt was torn away. The 



