THE ORIGIN AND DESCENT OF ROCKS. 485 



constituent. Once deposited on the walls ores aid, by mass action, the 

 further accretion of ores. 



The special forms which ores assume in deposition, as beds, veins, 

 lodes, stockworks, disseminations, segregations, etc., are chiefly inci- 

 dental to the local situation in which the essential chemical or physical 

 change takes place. 



