LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 45 



and, doubtless, 



NaOH+HCl ^NaCl + H^O 

 2NaOH+H.S ^Na^S +2 H^O 

 2NaOH+CO. ^Na.C03+H.O 



with similar reactions for the corresponding potash compounds, 

 besides very complicated equilibria between the molecules S, SO2, 

 SO3, C, CO, CO., H.S, H, H.O, O, HCl, CI, etc. 



Though we understand very Kttle about the exact form in which 

 iron and magnesia enter into the micas, it appears that there is the 

 same tendency to partial breakdown from the more siliceous 

 metasilicate to the less siliceous orthosilicate. 



There is no doubt that the increased concentration of water in 

 the magma at this stage exerts a strong influence in promoting this 

 breakdown of the polysilicate molecules of the alkalis and the meta- 

 silicates of iron and magnesia into the orthosilicate molecules with 

 setting free of silica, an action which may be compared with 

 hydrolysis. Niggli^ ascribes such action to water in discussing the 

 rocks of Electric Peak and Sepulchre Mountains described by 

 Iddings.^ In some of the deep-seated rocks of Electric Peak quartz 

 and biotite occur, whereas they are absent in surface rocks of the 

 same composition at Sepulchre Mountains. This is ascribed to 

 loss of water by the surface rocks. Niggli states: "Zugleich hat 

 dieses Wasser offenbar einen Teil der Kieselsaure aus den Ver- 

 bindungen gedrangt und so in der Tiefenfazies zu freiem SiO. 

 gefiihrt." It is also well known that a hornblendic rock without 

 quartz may, even after complete solidification, be changed to a 

 biotite-quartz rock during metamorphism, when there is special 

 activity of volatile constituents, including water. An example has 

 been described by Cushing.^ 



It is not to be imagined that any of these reactions in the 

 magma begin abruptly at any special stage in the history of the 

 magma. For any given concentration of the molecules KAlSisOg 



' "Die gasformigen Mineralizatoren im Magma," Geologische Rundschau, Band 

 III (1912), 479. 



^ U.S. Geol. Survey, 12th Ann. RepL, I, (1891), 657; Iddings, Igneous Rocks, I, 

 152-53- 



3 " Geology of the Thousand Islands Region," N.Y. State Museum, Bull. 145, p. 40. 



