420 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



confined to the glassy groundmass. In the rather uncommon occurrences 

 just noted, where a cracking has been followed by crystallization, it is 

 probable that the shrinkage and cracking took place in a highly heated, 

 stiff mass, which was subsequently welded together, as the collapsed pumice 

 may have been — that is, it may have been surrounded by a hotter portion of 

 the same lava flow and its temperature may have been raised to some extent. 

 Instances of this kind of structure are found on the continental divide 

 south of Madison Lake and on the summit of the west wall of Bechler 

 Canyon, 5 miles from its mouth (1945, 1946), and also in the vicinity of 

 the Lower Geyser Basin. 



From the foregoing it appears that certain forms of crystallization 

 which unquestionably take place in molten magmas at or near their jjoint 

 of solidification, and which may be classed as pyrogenous and primary, 

 may take place in a mass at a time subsequent to the development of fea- 

 tures which seem to be dependent upon a certain amount of rigidity of 

 the magma, such as the formation of cracks. Such rigidity is generally 

 supposed to indicate perfect solidification and completed pyrogenous 

 crystallization; and undoubtedly it does in most cases. But rigidity has 

 a relative significance, and what is rigid with respect to a force acting 

 through an extremely short period of time may be plastic toward the same 

 force acting through longer time. Hence a highly viscous magma may 

 be torn to fragments by an explosion, or be highly inflated by the sudden 

 expansion of vapor, and in some cases be still viscous. Generally, however, 

 the sudden expansion and escape of inclosed vapors tend to lower the tem- 

 perature of the magma and increase its viscosity. But it may not necessa- 

 rily be solid or rigid. Similarly, shrinkage cracks may be produced in a 

 viscous mass by sudden contraction before the mass has solidified and while 

 it is still highly heated, the fracturing being due to the rapidity of change 

 of volume, and not necessarily to the absolute amount of contraction. Such 

 sudden contraction of unsolidified lavas is not of common occurrence, it 

 would seem, but the instances of postperlitic spherulitic crystallization just 

 described indicate its occasional occurrence. 



In the massive glasses which are striped and marked with bands of 

 various color it is sometimes noticed that lines of color traverse the mass 

 like ancient fractures which closed up before the solidification of the mass. 

 They pass across the lines of flow, which sometimes end abruptly against 



