DEVITRIFICATION OF GLASS. 251 



originally partly or wholly glassy. For instance, the Berlin gneiss of 

 central Wisconsin is in chemical composition the same as that of various 

 associated aporhyolites. The aporhyolites show that they were originally 

 glasses by retaining the characteristic textures of glass. The Berlin gneiss 

 which was altered under conditions of mashing in the deep-seated zone is 

 entirely devoid of any structure which can be attributed to glass, and one 

 can not be certain that it did originally have a glassy base, although this 

 seems probable. 



Minerals produced. — The minerals which are produced by the alterations of 

 glass are very numerous. It has already been noted that glasses form 

 from the most acid magmas, and also from those which are intermediate or 

 basic in character. Furthermore, it has been seen that glass is devitrified 

 in both the upper and the lower physical-chemical zones, and in the upper 

 zone both in the belt of weathering and in that of cementation. In each of 

 these zones and belts minerals form from the glass which are characteristic 

 of them. It is plain from the foregoing that every mineral which may be 

 a metamorphic product of an igneous rock of any kind may result from the 

 devitrification of glasses of different kinds under the different conditions 

 which obtain in the zones and belts of alteration. 



Heat and volume relations. — The devitrification of glass is a process which 

 probably results in the liberation of heat. This is certainly true for the 

 zone of katamorphism, where oxidation, hydration, and carbonation take 

 place. As to the volume relations of the change, the devitrification itself by 

 means of which the substance passes from an amorphous to a crystalline 

 condition would decrease the volume, provided there were no additions of 

 other compounds. But where devitrification is accompanied by oxidation, 

 carbonation, and hydration there are considerable additions of material. 

 Therefore, in the belt of cementation there can be little doubt that 

 expansion of volume is the rule where glasses are devitrified; but in the 

 belt of weathering, where solution is prominent, doubtless there is diminution 

 in volume with glass as with other compounds. In the zone of anamorphism 

 devitrification takes place with decrease of volume, the reactions being 

 controlled by pressure. Whether heat be liberated or absorbed in the zone 

 of anamorphism doubtless depends in large measure upon how far the 

 reactions of the zone of katamorphism have taken place during the time 



