484 GEOLOGY. 



tributed, and lateral segregation results. At the same time the atmos- 

 pheric waters acting at or near the surface concentrate ore values down- 

 wards. The sum total of these processes is to promote the development of the 

 higher ore values in accessible horizons, and along the main lines of circula- 

 tion. 



The influence of contacts. — As ore-deposits depend on a dissolving 

 state followed by a depositing state of the waters, and perhaps on a com- 

 plex succession of these alterations, it is obvious that conditions which 

 favor changes of state and the commingling of different kinds of water 

 are apt to be favorable to ore production. At any rate it is observed 

 that many important ore -deposits occur at the contact between forma- 

 tions of different character. The contact of igneous rock with Hmestone 

 is a rather notable instance. It is not to be inferred that such contacts 

 are generally accompanied by workable ore-deposits, but merely that 

 a notable proportion of workable ore -deposits occur at such junctions. 

 It is rational to suppose that where the chemical nature of the two 

 formations is in contrast, the waters that percolate through the one 

 are hkely to be minerahzed very differently from those that course 

 through the other, and hence that on mingUng at the contact, reactions 

 are specially Hable to take place, and that when a valuable metalUc 

 substance is present it is Hable to be involved and by chance to suffer 

 precipitation. Reactions are the more probable because the contact 

 is hkely 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 facihtate circulation and offer suitable places for ore 

 formation. 



The effect of igneous intrusions.— A special case of much impor- 

 tance arises when lavas are intruded into sediments that have previously 

 been partially enriched in the ways above described. The igneous intru- 

 sion 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 differ- 

 ent temperatures. The special efficiency of these agencies is beUeved 

 to be the determining factor in many cases. 



The influence of rock walls.— The rock walls themselves are thought 

 sometimes to be a factor in ore-precipitating reactions. By mass action, 

 they may withdraw a constituent of the solution and destroy its equilib- 

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



