244 J- ^' L' ^OGT 



In general, we may set up the thesis: Compositions of eutectic 

 or almost eutectic character further the formation of glass. This 

 principle really gives the general foundation for glass technology. 

 At a medium rate of cooling of molten pure CaSiOa, of pure Na2Si03, 

 or KzSiOj, respectively, crystallization takes place. A eutectic 

 or nearly eutectic mixture of CaSiOj and NazSiOa, respectively 

 KzSiOj, gives, on the other hand, with fairly quick cooHng, glass 

 only, without any formation of crystals. If also, as in the manu- 

 facture of glass, a third component, SiOz, is added, the viscosity is 

 considerably increased even at high temperatures. On cooling a 

 melt nearly of the composition of the ternary eutectic CaSiOa! 

 NazSiOj-.SiOa to a point on the melting surface (or in view of the 

 supersaturation somewhat below the melting surface), the viscosity 

 is so considerably increased that with medium quick cooHng no 

 formation of crystals can take place, i.e., glass results. 



The principle here treated has a considerable influence on the 

 structure of the geologically, relatively quickly cooled dikes and 

 flows. By progressive crystallization at decreasing temperatures, 

 the viscosity of the residue melt generally increases. The conse- 

 quence is a fine-grained ground mass, or with still quicker cooling, 

 a total restraint of crystallization, so that the remainder soHdifies 

 as glass. Melts rich in iron and basic (with about 50 per cent 

 SiOz) are not nearly so viscous as those poor in iron and acid (with 

 about 68-75 P^^^ cent SiO^). Because of this, glass basis usually 

 plays a more important part in the quickly cooled acid rocks than 

 in the basic. 



Melts of anchi-eutectic composition poor in H2O maybe extremely 

 viscous at the relatively low temperature of crystallization which 

 characterizes them. With relatively quick coohng, consequently, 

 the crystalHzation will be entirely or nearly entirely restrained. 

 Thus it is no accidental circumstance that by far the most of 

 obsidians have nearly the chemical composition of the granitic 

 eutectic. Many, perhaps even the most, obsidians contain 72-76 

 per cent SiOz and between 0.3 Or:o.7 Ah+An and 0.5 Or:o.5 

 Ab -i-An. They thus even approach the " ternary " granitic eutectic, 

 having an especially low temperature for the interval of crystalliza- 

 tion, and thus also an especially high viscosity at the stage where 



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