180 A TREATISE ON METAMORPHISM. 



stated." In the zone of katamorpliism the complex hydrous silicates, such 

 as the kaolins, serpentines, and zeolites form. In the zone of anamorphism 

 these minerals are largely dehydrated, and such minerals as muscovite, 

 andalusite, garnet, staurolite, etc., are produced. 



The physical-chemical principles cited (pp. 45-123) give reasons for 

 the existence of the above reverse sets of reactions in the two zones. We 

 can now give chemical or physical causes why oxidation, carbonation, and 

 hydration take place in the zone of katamorpliism, and deoxidation, 

 silication, and dehydration in the zone of anamorphism, and so on for other 

 reactions. 



While each of these sets of processes is particularly characteristic of 

 one zone, it is not meant to imply that each reaction may not occur in 

 both zones. But in the zone of katamorpliism, oxidation, carbonation, 

 and hydration greatly predominate over the reverse processes. On the 

 other hand, in the zone of anamorphism, deoxidation, silication, and dehy- 

 dration predominate over the reverse processes. 



If all of these sets of processes reversed as preponderant reactions at 

 the same depth, it would be possible to sharply separate the zones of 

 katamorpliism and anamorphism. If, for instance, for a given region above 

 a depth of 10,000 meters the sum totals of the oxidation, carbonation, and 

 hydration were greater than the sum totals of reverse processes, the zone 

 of katamorphism would be sharply separated from the zone of anamor- 

 phism at this depth. But this is not the case. The reversal of each pair 

 of processes occurs at different depths; and, further, the reversal for a 

 given pair of processes is at different depths under different conditions. 

 One of the most important of these is as to whether the conditions are 

 mass-static or rnass-mechanical. 



Of the three sets of reversing reactions, oxidation and deoxidation, 

 carbonation and silication, hydration and dehydration, the first reverses 

 with the. least depth and pressure, the second requires the greatest depth 

 and pressure, and the last a mean depth and pressure. It has already been 

 noted that oxidation very frequently is replaced by deoxidation in the 

 lower part of the zone of katamorpliism. It is certain that the process of 

 hydration is very greatly stayed, if it does not altogether cease, and may 



"Van Hise, C. R., and Bayley, W. S., The Marquette iron-bearing district, of Michigan: Mon. U. S. 

 Geol. Survey, vol. 28, 1897, pp. 444-459. 



