CHEMICAL WORK IN ZONE OF ANAMORPHISM. 679 



to get. average determinations, this might enable one to arrive at alternative 

 conclusions as to the pressures and temperatures under which the alterations 

 took place. If a given temperature were assumed, the pressure, and there- 

 fore the depth, could be calculated. If, on the other hand, pressure were 

 assumed, the temperature could be calculated. 



Silication involves decrease in volume, varying from a very small 

 amount to 40 per cent or more. In the silication of the carbonates, if the 

 freed carbon dioxide be supposed to escape, the decrease in volume is 

 usually between 20 and 40 per cent, and averages fully 30 per cent. In 

 so far as there are cavities that are filled with the water and carbon dioxide 

 which have been liberated by the processes of silication and dehydration 

 but have not escaped, the above determinations as to diminution of volume 

 are too great. No estimates of the volumes of these inclusions in the meta- 

 morphic sedimentary rocks have been made, but the average amount would 

 probably be comparatively small, possibly less than 1 per cent, although 

 in certain of the schists and gneisses the amount would be much greater 

 than this. 



DEHYDRATION. 



It has been seen in the previous chapters (Chapters IV to VII) that 

 hydration is one of the most important and characteristic of the^ reactions 

 which occur in the zone of katamorphism. By that process alone or com- 

 bined with others a large number of hydrated minerals are formed. The 

 most important of these comprise the hydrous silicates and the hydrous 

 oxides. The hydrous silicates include the kaolin group, the serpentine-talc 

 group, the chlorite group, the hydromica group, the zeolite group, and the 

 epidote group. The oxides include the aluminum-oxide group, of which 

 gibbsite is the most important, and the iron-oxide group, of which limonite 

 is the most important. In the zone of anamorphism all these minerals are 

 dehydrated; but in general one can not assert that from a definite one of 

 them some other definite mineral is produced, for commonly during the 

 time of dehydration other alterations also take place, and commonly the 

 materials of two or more minerals unite to produce a new mineral. How- 

 ever, in some cases the alteration is that of simple dehydration; as, for 

 instance, hematite is formed from limonite, anhydrite is produced from 

 gypsum, quartz is formed from opal, corundum develops from gibbsite. 

 By the process of simple dehydration there is usually a very considerable 

 decrease in volume, running from 20 to 40 per cent. 



