DEPTH OF OXIDATION AND SOLUTION. 561 



These iron-ore deposits consist mainly of hematite with some limonite, and 

 occasionally some magnetite, with silica as the main impurity. In passing- 

 downward the silica is likely to become somewhat more abundant, and 

 finally the ores become too lean for working. As explained in another 

 place, these ores have been precipitated in pitching troughs on impervious 

 basements by downward-percolating waters which bear oxygen, often at 

 places where the rocks have been much fractured by orogeiiic movement 

 and are therefore very open and porous. Simultaneously with the pre- 

 cipitation of the iron oxide silica is dissolved. (See pp. 1193-1197.) It 

 appears, therefore, in the case of these ore deposits, that oxidation and solu- 

 tion, both reactions characteristic of the belt of weathering, have locally 

 extended for 300 meters or more into the belt of cementation. 

 HON xlvii — 03 36 



