4 FROM FISH TO PHILOSOPHER 



tinents, which are composed largely of granite, are 

 lighter than the underlying mantle and consequently 

 they Hterally float on the surface of the mantle as ice- 

 bergs float in the ocean, with the greater part of their 

 mass, particularly under the moimtain ranges, sub- 

 merged to depths of 15 to 40 miles. At the center of the 

 earth is a very dense core, thought to consist of iron and 

 nickel. Even these general statements, which are based 

 chiefly on the velocity of transmission of earthquake 

 waves, are matters of debate: some geophysicists refer 

 to an inner and outer core which differ in density; the 

 composition of the mantle, and particularly the depth 

 of the basaltic crust, is a matter of divided opinion; and 

 the composition of the outermost crustal layer underly- 

 ing the sea basins is not certainly known. The transition 

 from the crust to the outer layer of the mantle is fairly 

 abrupt and is known as the Mohorovicic discontinuity, 

 and familiarly referred to by geophysicists as the Moho. 

 Geophysicists hope to explore the composition of the 

 outer mantle in the future by boring through the crust 

 where it is thiimest imder the ocean. 



It is more certain, however, that the pressure inside 

 the earth increases with depth: at the bottom of the man- 

 tle the pressure is estimated to exceed 1 million atmos- 

 pheres, at the center of the earth about 4 miUion atmos- 

 pheres. The density increases with increasing pressure: 

 the greatest density of rocks at the surface is about 3, 

 at the bottom of the mantle perhaps 5.6, and the me- 

 tallic core may have a density of 10 to 12; the average 

 density of the earth is calculated by astronomers to be 

 only 5.5, a quite reUable figure which enables the geo- 

 physicist to bring his seismologic and other data into a 

 balanced account. As one bores vertically into the earth 

 the temperature increases about 16° F. for every thou- 

 sand feet, so that in deep wells the boiling point of water 

 is reached at about 7000 feet; it is in this region that 

 the steam is formed which actuates hot geysers. Assum- 

 ing a uniform rate of increase, the temperatures of mol- 



