1068 A TREATISE ON METAMORPHISM. 



fact that hot waters deposit more minerals and different minerals an evidence 

 that the hot waters are derived from crystallizing- igneous rocks, although 

 it is possible, and rather probable, that in many instances igneous rocks 

 have contributed materials to the solutions. The high mineral content 

 of waters is adequately explained as a function of high temperature. 

 That this is certainly true for Mammoth Hot Springs has already been 

 shown. (See pp. 1031-1032.) The view that the waters of Boulder Hot 

 Springs and of the Yellowstone Park Springs are of meteoric origin was 

 held by Weed at the time he studied and reported upon these thermal 

 regions." 



Adjacent to great intrusive igneous masses the contributions of water 

 from crystallizing magma may be important. Since in many cases the ores 

 are deposited during periods of volcanism, the exuding waters are hot, and 

 may be charged with mineral material to an unusual extent. In such 

 cases the waters derived from igneous sources are proportionately far more 

 active than ordinary underground waters. In many instances the water 

 liberated by the magma during solidification has doubtless acted as a potent 

 agent in segregating the ores, but after solidification waters of meteoric 

 origin continue the work. Often to a large extent the solution of the metal 

 and its deposition takes place after the magma has crystallized, but while the 

 rocks are still hot. After an igneous rock has solidified and continues to 

 cool the ordinary process of shrinkage tends to form openings within it, 

 and especially to form openings along its contact with the contiguous 

 deposits. Orogenic movements also find that the contacts between the 

 igneous and the adjacent rocks are planes of weakness, and are likely there 

 to form openings. This being the fact, it is entirely natural, indeed inevi- 

 table, that the hot circulating waters during the time that the rock is cooling 

 are most active in the segregation of the deposits. The extreme activity of 

 hot water in general metamorphism and in the production of ore deposits 

 has been repeatedly emphasized. Thus in various instances the water 

 liberated by the magma at solidification may have played an appreciable 

 part in the segregation of ores, but very often the importance of this part 

 has been overemphasized because it has not been appreciated that after 

 solidification takes place the solutions continue the work of seg*reg*ation of 

 the ores 'under most favorable circumstances. 



a Weed, Walter Harvey, Mineral vein formation at Boulder Hot Springs, Mont. : Twenty-first 

 Ann. Rept. V. S. Geol. Survey, pt. 2, 1900, pp. 249-252. 



