186 THE LURE OF THE LAND 



is considered there may be a tendency to despise their 

 importance, but their continuity and their generality 

 in the opinion of Miintz place them among the geo- 

 logical causes to which the crust of the earth owes a 

 part of its actual physiognomy and which particularly 

 have contributed to the formation of the deposits of the 

 comminuted elements constituting arable soil. 



" Brauner calls attention to the danger of overesti- 

 mating the activity of nitrifying organisms in effecting 

 the decay of rocks due to their inability to live at great 

 depths. As is well known they diminish in abundance 

 as the depth below the surface increases and disappear, 

 or at least are inactive, at depths of from three to six 

 feet. Brauner says that in these cases the statements 

 of specialists are the only safe reliance and the finding 

 of bacteria in rocks by any one not a specialist is to be 

 regarded with suspicion. Only those who have worked 

 in bacteriology can fully appreciate the difficulties to 

 be enumerated and the precautions to be taken in deal- 

 ing with those organisms in order to prevent being mis- 

 led by faulty manipulation. 



THE ROLE OF OXYGEN. 



" The air itself takes an active part in rock decay. 

 Wherever rocks are exposed to decay, there air is found 

 or, at least, the active principle of air, viz., oxygen. 

 The air as a gas not only penetrates to a great depth 

 in the earth, but is also carried to much greater depths 

 by water which always holds a greater or less quantity 

 of air in solution. The oxygen of the air is thus 

 brought into intimate contact with the disintegrating 

 materials and is present in a condition to assist wher- 

 ever possible in the decomposing processes. 



" The oxygen acts vigorously on the lower oxids of 



