6 FROM FISH TO PHILOSOPHER 



Despite the long period during which the earth has 

 had a hard crust, it has never acquired a truly rigid 

 structure. This is partly because of the high tempera- 

 tures of the interior, and partly because of the great pres- 

 sures to which the interior is subjected— for example, at 

 a depth of only 400 miles the pressure is 8,000,000 

 pounds per square inch. All solid bodies when placed 

 under pressure behave like viscous fluids, and the earth 

 as a whole has this semifluid character. This 'fluidity' 

 is such that the familiar oceanic tides raised by the 

 moon's gravitational force are paralleled by similar tides 

 in the earth's crust, each circuit of the moon raising and 

 lowering continents and seas, deserts and mountain 

 ranges, by some 16 inches. 



According to an interpretation first proposed by Sir 

 George Darwin (son of Charles Darwin), the moon was 

 literally torn out of the earth by a solar tide of this na- 

 ture; this fission hypothesis goes far to explain several 

 otherwise peculiar facts, but contemporary cosmogonists 

 favor the belief that the earth and moon were formed 

 simultaneously. In any case, as George Gamow has 

 pointed out, if one assumes a uniformly thick crust of 

 granite the volume of water in the oceans is sufficient to 

 cover the entire earth to a depth of several miles. Were 

 it not for irregular distribution of granite in the conti- 

 nents, the evolution of terrestrial life could never have 

 occurred. Throughout most or all of geologic time the 

 surface temperature of the globe has been below the 

 boiling point of water and its surface therefore largely 

 covered by water. 



Scarcely less plastic than the interior of the earth are 

 the granite continents themselves, which throughout 

 earth history have been repeatedly warped and wrinkled 

 and at points actually overfolded in moimtain chains. 

 The causes of orogeny {oro = mountain; gignesthai = to 

 be bom) are multiple and extremely complex. Most im- 

 portant, perhaps, is the heat generated by radioactivity 

 in the interior: this accumulates for a time and leads to 



