948 A TREATISE ON METAMORPHISM. 



deposits, as for instance in the case of iron. The development of the greater 

 number of ore deposits involves the segregation of the rarer elements, and 

 this aspect of the subject is especially considered in the following chapter. 



Oxygen is the most abundant of the elements, both in the lithosphere 

 and in the hydrosphere. According to Clarke's estimate of 1891 oxygen 

 composes 49.98 per cent of the lithosphere, hydrosphere, and atmosphere 

 together, or only 0.04 per cent short of the total amount of all the other 

 elements. According to Clarke's estimate of 1891 it composes 47.29 per 

 cent of the lithosphere, and according to his estimate of 1900, 47.02 per 

 cent." Of the hydrosphere it composes 85.79 per cent, and of the atmos- 

 phere 23.12 per cent. While the percentage of oxygen in the hydrosphere 

 is greater than in the lithosphere, by far the greater proportion of the 

 oxygen is in the lithosphere, on account of its enormous mass, although an 

 immense quantity is in the hydrosphere. The oxygen of the lithosphere 

 and hydrosphere is combined, while that of the atmosphere is free. 

 Oxygen differs from all of the other abundant elements in that a large 

 quantity of it is in the free state. The only other abundant element which 

 occurs partly in the free state is iron, and the amount of free iron in the 

 lithosphere is exceedingly small. One other element, nitrogen, near the 

 bottom of the list of the 23 more plentiful elements, also occurs in the free 

 state and in even greater quantity than oxygen. Oxygen is the element 

 second in abundance in the meteorites. Among the gases of the meteorites 

 free oxygen is not included in the list given by Farrington, but Wright 

 reports oxygen as occluded.'' Even if oxygen exists as a gas in meteorites, 

 this is not evidence that this oxygen was brought from the outer sj)ace_ 

 It may have been absorbed there or while the meteors were passing through 

 the atmosphere, or partly in both ways. 



The source of oxygen for the atmosphere is a matter of great conse- 

 quence, since in the process of oxidation, one of the fundamental processes 

 of the zone of katamorphism, the oxygen is derived directly or indirectly 

 from the atmosphere. So far as known, the vast quantity of free oxygen 



"Clarke, cit., Bull. 78, p. 39: Bull. 168, p. 15. 



6 Wright, A. W., Spectroscopic examination of gases from meteorites: Am. Jour. Sci., 3d ser., vol. 

 9, 1875, p. 301. 



