II. THE CHEMICAL ELEMENTS CONSTITUTING 



ROCKS 



Althougli there are upwards of seventy elements now known, 

 but sixteen occur in any abundance or form more than an ex- 

 tremely small proportion of the material of the earth's crust. In- 

 deed, of this number probably fully one-half, taken collectively, 

 will not constitute more than 4 or 5% of the earth's crust so far 

 as known. These sixteen, arranged according to their chemical 

 properties and the order of their abundance, are as follows: 

 oxygen, silicon, carbon, sulphur, hydrogen, chlorine, phosphorus, 

 fluorine, aluminum, calcium, magnesium, potassium, sodium, 

 iron, manganese, and barium. The eight more important, with 

 their approximate percentage amounts as shown by the most 

 recent calculation,^ are as follows : 



Calehim 3.50 



Magnesium .... 2.62 



Sodium 2.63 



Potassium .... 2.32 



Oxygen . . . 



. . 47.02 



Silicon . . . 



. . 28.06 



Aluminum . . 



. . 8.16 



Iron .... 



. . 4.64 



It must not be imagined, however, that these elements exist 

 for the most part in a free or uncombined state: on the con- 

 trary, in the majority of cases so great is their affinity for one 

 another that it is only momentarily, or under abnormal con- 

 ditions, that they are met with at all in this elementary form. 

 The elements which are most common in the free state, though 

 even these occur more commonly combined with others, are, 

 (1) the gas oxygen, and (2) the solids, carbon, sulphur, and, 

 more rarely, iron. Still more rarely, and under such abnormal 

 conditions, as exist during volcanic eruptions, are found the 

 free gases, hydrogen, chlorine, and, according to some authori- 

 ties, fluorine. The gas nitrogen, although so abundant a con- 

 stituent of the atmosphere, is, as a primary constituent of the 



^ By F. W, Clarke, Bull. 168, U, S. Geol. Survey, 1900. The figures given 

 in tlie first edition of the present work as quoted from Boscoe and Sehor- 

 lemmer's Treatise on Chemistry, were: Oxygen 44.0-48.7, Silicon 22.8-36.2, 

 Aluminum 9.9-6.1, Iron 9.9-2.4, Calcium 6.6-0.9, Magnesium 2.7-0.1, Sodium 

 2.4-2.5, Potassium 1.7-0,1. 



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