980 A TREATISE ON METAMORPHISM. 



NITROGEN. 



Nitrogen forms 0.02 per cent of the outer 10 miles of the crust of the earth 

 including the lithosphere, hydrosphere, and atmosphere." All but an inesti- 

 mably small quantity of this is in the atmosphere, of which it composes 

 76.88 per cent by weight. (See p. 944.) It stands sixteenth in the scale of 

 abundance. Nitrogen is among the gases which are occluded in meteorites; 

 but what portion of this nitrogen is absorbed by the meteorites while passing 

 through the atmosphere and what portion has been brought in from the 

 interplanetary spaces is uncertain. The amount of nitrogen in the average 

 sedimentary rocks, such as shales, sandstones, and limestones, is so small 

 as to be inestimable. 



It has been explained that in consequence of the action of bacteria and 

 leguminous plants nitrogen is fixed in the belt of weathering, and nitrates 

 are formed. These nitrates in arid regions are locally segregated in the 

 belt of weathering in considerable amounts, forming the nitrate deposits. 

 The bases with which the nitrogen is combined are almost exclusively 

 sodium and potassium. Since the nitrates are now forming in large quan- 

 tities (see pp. 452-453, 465-466) the total amount of the compounds which 

 have been produced through geological time must have been large. But the 

 nitrates readily break up into their original elements, and the liberated 

 nitrogen and oxygen rejoin the atmosphere. Thus there is a continuous 

 cycle by which nitrogen, oxygen, and carbon are united by means of the 

 leguminous plants and bacteria; and by decomposition reduced to the 

 original elements and carbon dioxide. 



While the total quantity of nitrates is inconsiderable, as compared with 

 the majority of the other important elements concerned in metamorphism, 

 nitrogen is essential for the growth of plants and animals. Hence nitrogen 

 is one of the necessary elements in the sequence of events by which 

 carbon is abstracted from the atmosphere, passes into the plants, anc [ 

 thence by oxidation is concentrated so as to carry on the process of car- 

 bonation. Therefore a chief importance of nitrogen in metamorphism is as 

 one of the aids in the chain of reactions in connection with carbon. 



The nitrates in the soil and the nitrate deposits represent the economic 

 products resulting from the abstraction of nitrogen from the air and its 

 segregation in solids. 



a Clarke, cit, Bull. 78, p. 39. 



