DEVITRIFICATION OF GLASS. 249 



(3) Pressure produces a state of unequal strain, and hence is favorable 

 to devitrification. 



(4) Mass-mechanical action not only produces a state of unequal 

 strain in minerals, but fractures the material, and this gives a large surface 

 of action for the solutions. It is therefore clear that mass-mechanical action 

 is very favorable to devitrification. 



(5) Experiments in the laboratory show that if glass be raised to a 

 temperature short of fusion the tendency to devitrification is greatly 

 promoted. It is therefore certain that conditions of dry heat after solidifi- 

 cation are favorable to devitrification. As glass occurs in considerable 

 bodies in a state of nature, it must for a long time, perhaps hundreds ot 

 thousands of years, have a high temperature due to the residual heat of 

 the magma, and only very gradually assumes the normal temperature 

 corresponding with its depth of burial. It is rather probable that micro- 

 lites and crystallites, which so frequently occur in glass, largely form 

 during this process of cooling after solidification. 



(6) While devitrification of glass may occur without the presence of 

 abundant water, it is probably rare indeed that in nature the process occurs 

 without the presence of some moisture, and in general moisture is a very 

 important factor favorable to devitrification. 



It is therefore clear that each of the above factors may give a condi- 

 tion favorable to devitrification, but in general actual devitrification is 

 due to a combination of two or more of them. 



Devitrification in the two zones. — In the zone of katamorphism under ordinary 

 conditions it is probable that the devitrification occurs somewhat slowly. 

 But in areas of regional volcanism, and often in those of local volcanism, 

 the lava flows follow one another in such rapid succession that beds are 

 piled up so deep that the water is held at a high temperature. By complex 

 intrusion the entire mass of a cooled glass may again be raised to a high 

 temperature. Oogenic movement if severe may produce a high tempera- 

 ture. Under any of these circumstances the conditions are furnished for 

 the complete and rapid devitrification of the glass. 



The nature of the devitrification is certainly different in the belt of 

 weathering and the belt of cementation, although available descriptions do 

 not furnish data for accurate statements as to the differences. But it is 

 certain that in the belt of weathering the several changes are along the 



