NATURE OF THE MATERIALS IN THE LITHOSPHERE 23 



The earth a magnet. The denser, and likewise the more 

 common, of the meteorite rocks the so-called meteoric irons 

 are composed almost entirely of the elements iron, nickel, and 

 cobalt. Such aggregates are not known as yet from terrestrial 

 sources, although transitional types appear to exist upon the 

 island of Disco off the west coast of Greenland. If it were pos- 

 sible to explore the earth's interior, would such combinations of 

 the iron minerals be encountered? Apart from the surprising 

 velocity of transmission of earthquake waves, the strongest argu- 

 ment for an iron core to the lithosphere is found in the magnetic 

 property of the earth as a whole. The only magnetic elements 

 known to us are those of the heavy meteorites iron, nickel, 

 and cobalt, and the earth is, as we know, a great magnet whose 

 northern pole in British America and whose southern pole in 

 Antarctica have at last been visited by Amundsen and David, 

 respectively. The specific gravity of iron is 7.15, and those of 

 nickel and cobalt, which in the meteorites are present in relatively 

 small amounts, are 7.8 and 7.5, respectively. Considering that 

 the surface shell of the earth has a specific gravity of 2.7, these 

 values must be regarded as agreeing well with the determined 

 density of the earth (5.6) and the other planets of its group (Mer- 

 cury 5.7, Venus 5.4, Mars 4). 



The chemical constitution of the earth's surface shell. The 

 number of the so-called chemical elements which enter into the 

 earth's composition is more than eighty, but few of these figure 

 as important constituents of the portion known to us. Nearly 

 one half of the mass of this shell is oxygen, and more than a quarter 

 is silicon. The remaining quarter is largely made up of aluminium, 

 iron, calcium, magnesium, and the alkalies sodium and potassium, 

 in the order named. These eight constituent elements are thus 

 the only ones which play any important role in the composition 

 of the earth's surface shell. They are not found there in the free 

 condition, but combined in the definite proportions characteristic 

 of chemical compounds, and as such they are known as minerals. 



The essential nature of crystals. A crystal we are accus- 

 tomed to think of as something transparent bounded by sharp 

 edges and angles, our ideas having been obtained largely from the 

 gem minerals. This outward symmetry of form is, however, but 

 an expression of a power which resides, so to speak, in the heart 



