OXYGEK 5 



earth's crust, almost wholly unknown, and needs no considera- 

 tion at this stage of our work. 



Oxygen, as is well known, is the active, even the aggressive, 

 principle of the atmosphere, of which it constitutes about one- 

 fifth by bulk. Combined with other elements, it is of great 

 geological importance, being estimated, as noted above, to con- 

 stitute 47.02% of the entire mass of the earth's crust; that is 

 to say, could this crust be resolved into its original elements, 

 the oxygen thus liberated would be found very nearly equal to 

 all the other elements taken together. The simpler forms of 

 oxygen compounds are known as oxides, and of these the oxide 

 of hydrogen, water (HgO), is by far the most common, and, 

 anomalous as it may at first seem, is a true mineral. Aside from 

 being so essential to human life, oxygen is a very potent factor 

 in the manifold changes which are constantly taking place in 

 the more superficial portions of the earth's crust. 



Silicon. — Next to oxygen silicon is the most abundant of 

 the earth's constituents, though it exists only in combination, 

 either as an oxide (silica), or with other elements to form 

 silicates. In these two forms it is the predominating con- 

 stituent in all but the calcareous rocks. As silica (SiOs), or 

 quartz, it forms one of the most indestructible of natural com- 

 pounds, and hence is to be found as the prevailing constituent 

 in nearly all sands and soils. 



Aluminum is next to oxygen and silicon probably the most 

 important element when regarded from the present standpoint. 

 It occurs mainly in combination with silicon and oxygen, form- 

 ing an important series of minerals known as aluminous sili- 

 cates. As a sesquioxide it is well known in the minerals 

 corundum and beauxite. 



Iron, although less abundant than either oxygen or silicon, 

 occupies a very important place as a rock constituent, owing to 

 the variety of compounds of which it forms a part, as well as 

 to the decided colors which are characteristic of its oxides and 

 of the iron-bearing silicates. The most conspicuous forms of 

 iron on the immediate surface of the earth are the oxides, but 

 which at greater depths, or where the atmosphere has as yet 

 exercised less influence, give way to carbonates, sulphides, and 

 silicates. 



Iron, although so common in combination with other elements, 

 occurs but rarely free, owing to its affinity for oxygen. 



