NATURE OF THE MATERIALS IN THE LITHOSPHERE 21 



A stronger proof of earth rigidity than either of these has been 

 lately furnished by the instrumental study of earthquakes. With 

 the delicate apparatus which is now installed for the purpose, 

 heavy earthquakes may be sensed which have occurred anywhere 

 upon the earth's surface, the earth movement sending its own 

 message by the shortest route through the core of the earth to the 

 observing station. A heavy shock which occurs in New Zealand 

 is recorded in England, almost diametrically opposite, in about 

 twenty-one minutes after its occurrence. The laws of wave 

 propagation and their relation to the properties of the transmitting 

 medium are well known, and in order to explain such extraordinary 

 velocity it is necessary to assume that for such impulses the earth's 

 interior is much more rigid than the finest tool steel. 



Probable composition of the earth's core. In deriving views 

 concerning the nature of the earth's interior we are greatly aided 

 by astronomical studies. The common origin long ago indicated 

 for the planets of the solar system and the sun has been confirmed 

 by the analysis of light with the aid of the spectroscope. It has 

 thus been found that the same chemical elements which we find in 

 the earth are present also in the sun and in the other stellar bodies. 

 Again, the group of planets of the solar system which are nearest 

 to the sun Mercury, Venus, the Earth, and Mars have each 

 a high density, all except Mars, the most distant, having specific 

 gravities very closely 5J, that of Mars being about 4. This 

 average specific gravity is also that of the solid bodies, the so-called 

 meteorites, which reach the surface of our planet from the sur- 

 rounding space. Yet though the earth as a whole is thus found 

 to have a specific gravity five and a half times that of water, its 

 surface shell has an average density of less than half this value, 

 or 2.7. 



The study of meteorites has given us a possible clew to the 

 nature of the earth's interior; for when both terrestrial and 

 celestial rock types are classified and placed in orderly arrange- 

 ment, it is found that the chemical elements which compose the 

 two groups are identical, and that these are united according to 

 the same physical and chemical laws. No new element has been 

 discovered in the one group that has not been found in the other, 

 and though some compounds of these elements, the minerals, oc- 

 cur in the earth's crust that have not been found in meteorites, 



