250 A TREATISE ON METAMORPHISM. 



lines given on pages 506-527 for that belt, finally resulting in the oblitera- 

 tion of textures and structures and producing an incoherent rock. 



In the belt of cementation ordinarily the alterations do not result in 

 the obliteration of the original textures and structures of the glasses. This 

 is sufficiently evident where the alterations occur under mass-static condi- 

 tions, and even where mass-mechanical conditions prevail. The glass is 

 simply fractured, as explained on pages 601-602, and the individual blocks 

 are altered by metasomatism under mass-static conditions. 



So far as we know, glasses originally form only in the zone of 

 katamorphism, and mainly at or near the surface. Therefore a glass can 

 get into the zone of anamorphism only by being buried under succeeding 

 lava flows or tuffs or under sedimentary rocks. Hence, before glass 

 reaches the lower zone, it must have been subjected for a long time to 

 devitrification in the belt of cementation, and the question arises whether 

 or not a glass would not be completely devitrified before it becomes 

 sufficiently deeply buried to reach the zone of anamorphism. However, if 

 glass ever does reach the lower zone, it is certain that its devitrification will 

 take place rapidly under either mass-static or mass-mechanical conditions. 

 The rocks in this lower zone are everywhere at temperatures exceeding 

 100° C; they contain water; hence, even under conditions of absolute 

 quiescence, it is certain that glass could not long exist. The crystallization 

 would be ev,en more rapid under mass-mechanical conditions. 



In so far as the glass had devitrified in the zone of katamorphism, and 

 had produced minerals characteristic of that zone, in the lower zone these 

 minerals would be recrystallized and minerals formed characteristic of the 

 latter zone. If mass-static conditions prevail this recrystallization may take 

 place without obliterating previous textures and structures. However, if 

 recrystallization takes place under conditions of mashing, the original 

 textures and structures are lost, and minerals are produced of such kinds 

 and proportions as correspond with the composition of the glass. More- 

 over, when the glass passes into the zone of anamorphism, textures and 

 structures may be formed characteristic of the slates, schists, and gneisses. 

 When such alteration is complete it is often impracticable to determine 

 whether the rock was originally glass or not. There can be little doubt 

 that many of the finer-grained schists are derived from rocks which were 



