98 IOWA ACADEMY OF SCIENCE 



ately evaluate. Meteoritic sources supply much larger amounts than it 

 has been customary to suppose. The part liberated by secular decay of 

 rock-masses is probably the largest. By the oxidation of small masses 

 of sulphide ores there is an appreciable contribution. Through the con- 

 stant working over of the bonanza layer the ores are kept localized and 

 concentrated. 



At any rate, deposition of ores derived from the vadose zone is very 

 much more important than is usually assumed. 



