74 GEOLOGY 



suffer precipitation. Reactions are the more probable because 

 the contact is likely to be a plane of crustal movement, and hence 

 more or less open and accompanied by fractures, zones of crushed 

 rock, and other conditions that facilitate circulation and offer 

 suitable places for ore formation. 



The effect of igneous intrusions. A special case of much im- 

 portance arises when lavas are intruded into sediments that have 

 previously been partially enriched in the ways above described. 

 The igneous intrusion not only introduces new contact zones, and 

 more or less fracturing, but it brings into play hot waters with 

 their intensified solvent work, their more active circulation, and the 

 reaction between waters of different temperatures. The special effi- 

 ciency of these agencies is believed to be the determining factor in 

 many cases. Furthermore the intruded lava may be rich in metallic 

 substances, and so be a favorable site for later concentration, and 

 the magmatic waters (waters from the lava) themselves appear to 

 be a source of important ore-deposits. The present tendency is 

 to attach more and more importance to the metallic contents of 

 magmatic waters. 



The influence of rock walls. The rock walls themselves are 

 thought sometimes to be a factor in the reactions which precipitate 

 ores. It appears that the effect of the wall may be to withdraw a 

 constituent of the passing solution, and destroy its equilibrium 

 in such a way as to cause the precipitation of the metallic constit- 

 uent. Once deposited on the walls, ore aids the further accretion 

 of ore of the same sort. The effect of the rock wall here noted is 

 sometimes called mass action. 



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

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

 incidental to the local situation in which the essential chemical or 

 physical change takes place. 



REFERENCE LIST OP THE MORE COMMON ROCK-FORMING MINER- 

 ALS, AND A FEW OF ECONOMIC IMPORTANT: 



Actinolite. A magnesium-caloium-iron amphibolc (q. v.); commonly 

 bright green to grayish green ; crystals usually slender or fibrous. 



Agate. A banded or variegated chalcedony quartz, (q. v.). 



