112 Z, V. Pirsson — ArlifiGial Lava-Flow 



In a sheet of molten glass the planes of cooling descend 

 vortifally into the mass so tliat variations of temperature from 

 point to })()int become much more marked in this direction 

 than in a hori/.ontal one. It is also true that in the cliange 

 from the li(piid to the solid condition a more or less consider- 

 able contraction of volume ensues. Further, the force of 

 crystallization is very powerful within the distance in which 

 it acts,* and in the final stage of viscosity before the capa- 

 bility of molecular movement ceases the tension on the 

 growing crystal faces must be very strong •a\\(\, per contra, on 

 the adjacent areas of unorientated molecules. Taking these 

 facts into consideration, it is clear that especially toward the 

 end of the process of spherulitic crystallization, these bodies 

 and the glass surrounding them will be subjected to stresses 

 which are most marked in vertical directions. Unable to with- 

 stand the tension, they may rupture, giving rise to horizontal, 

 or longitudinal, cracks or even be pulled apart so that ovoid, 

 or spherical, cavities are opened within them, as illustrated in 

 fig. 3. Or the tension between them and the surrounding- 

 glass may be relieved by tangential, or radial, cracks in the 

 latter, as also illustrated and described. Thus the layers of 

 spherulites spread through the glass in bands by ilowage become 

 elements of inherent weakness and along them it splits into 

 plates and thus has a laminated structure. This phenomenon 

 is also noticed in natural rhyolite glasses, like those from Lipari 

 and the Yellowstone Park, which cleave readily along the 

 bands of spherulites. Iddings,t in discussing the laminated 

 nature of such rocks, attributes it to non-homogeneity in dif- 

 ferent parts of the magma, produced by variable amounts of 

 contained water vapors, which unlike portions by the spread- 

 ing out action of flowing lavas become distributed in thin 

 sheets. Hence arise layers of different degrees of consistency, 

 crystalline character, etc., which condition the banded struc- 

 ture and cause lamination. The Kane glass, however, shows 

 that the same lamination, though not perhaps in so high a 

 degree, can be formed in the cooling of an anhydrous magma 

 and that the water vapor, except where it causes layers of bub- 

 bles, as in pumice, must be an indirect rather than a direct 

 agent in producing lamination in that it promotes more favor- 

 able conditions for crystallization. 



Differerdiation. — In the clear glass containing relatively 

 few small spherulites there is a non-homogeneous quality which 

 shows itself by drawn out streaked lines and layers, these 

 being nowise different in color or consistency, but having dif- 



* Becker and Day, liinear Force of Growing Crystals, Wash. Acad. Sci. 

 Proc, vol. vii, p. 283, 1905. 



f This Journal, vol. xxxiii, p. 48, 1887 ; Igneous Rocks, vol. i, p. 243, 1909. 



