ORE-DEPOSITS 



portance arises where lavas are intruded into sediments that have 

 previously been partially enriched in the ways described above. 

 The igneous intrusion not only introduces new contact zones, and 

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

 tln-ir intensified solvent work, their more active circulation, and the 

 reaction between waters of different temperatures. The special 

 efficiency of these agencies is believed to be important in many 

 cases. Furthermore the intruded lava may be rich in metallic 

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

 magmatic waters themselves appear to be a source of important 

 ore-deposits, . as already noted, and the present tendency is 

 to attach more and more importance to them. Ores deposited by 

 magmatic waters are, in a sense, the product of magmatic segrega- 

 tion (p. 42). 



The influence of rock walls. The rock walls themselves are 

 thought to be a factor, in some cases, in the reactions which pre- 

 cipitate ores. It appears that the effect of the wall may be to with- 

 draw some constituent of the passing solution, and destroy its 

 equilibrium in such a way as to cause the precipitation of metallic 

 constituents. Once deposited on the walls, ore aids the further 

 accretion of matter of the same sort. The effect of the rock wall 

 here noted is sometimes called mass action. 



The special forms assumed by ores deposited from solution 

 underground (veins, beds, etc.), are incidental to the local situation 

 in which the precipitation takes place. 



SUMMARY 



All in all, ground-water is to be looked upon as a most important 

 geological agent. When it is remembered that a very large part 

 of all the water which falls on the surface of the earth, either in the 

 form of rain or snow, sinks beneath the surface; that some of it 

 sinks to a great depth; that much of it has a long underground 

 urse before it reappears at the surface; that it is everywhere 

 and always active, either in subtracting from the rock through 

 which it passes, in adding to it, in effecting the substitution of one 

 mineral substance for another, or in bringing about new chemical 

 combinations; and when it is remembered that these processes have 

 been going on for untold millions of years, it will be seen that the 

 total result accomplished must be great. The rock formations of 

 the earth to the depths to which ground-water penetrates, are to 



