iKT. 17 MINERALS OF OBSIDIAN CLIFF — FOSHAG 17 



are, too, large and more perfectly developed. This siiiigests also 

 that iron and silica are comparatively more soluble under these con- 

 ditions than the feldspar. It is interesting to note that Niggli '* 

 found that crystals forming in the " pq region," that is, by direct 

 crystallization of the solid phase from the gaseous solution, were 

 man}' times larger than those formed from the liquid solution. 

 Larsen -'" noted that in the rocks of the Creede District the quartz 

 and orthoclase formed nests in a fine grained groundmass in which 

 these later minerals were enormously larger than those of the 

 groundmass, a condition again suggesting that the crystals were 

 formed from gaseous solution. 



RELATION OF WATER TO THE CRYSTALLIZATION OF FLOWS 



The effect of dissolved gases to reduce the viscosity of a melt and 

 thus to favor crystallization is already well known. The solvent 

 action of the escaping gases upon the glass, with its concomitant 

 separation of crystalline phases has not been emphasized as a means 

 of bringing about a crystalline mass. It seems probable that the 

 action of what appears at first sight to be an almost insignificant 

 amount of water vapor upon highly acidic rocks, like those of 

 Obsidian Cliff, can, under favorable circumstances, cause an almost 

 complete crystallization. There are several facts that seem to bear 

 out the importance of this action. First there is the prevalence of 

 tridymite and cristobalite in the acidic flow rocks, not only in the 

 cavities but in the groundmass as well. This fact was first clearly 

 brought out by Larsen.-*^ In his studies of the rocks of the Creede 

 District he showed that these two minerals were relatively more 

 important than quartz. The inconspicuousness of both in thin 

 section have made them easily overlooked and their presence is 

 probably more widespread than is generally believed for it is only 

 recently that the wide distribution of these mine^rals has been 

 recognized. The second fact is the characteristic structure of many 

 of the rhyolites and related rocks that manifests itself in a rough 

 appearance and feeling of harshness, due to the numerous small 

 cavities lined with tridymite and feldspar crystals. The well- 

 known tendency of anhydrous melts of silica and the alkali feld- 

 spars to form glass rather than to crystallize, suggests that the 

 mineralizers are a most important factor in bringing about crystal- 

 lization in acidic flows. It might be mentioned also that in some 

 of the most successful mineral syntheses, the action of water vapor 

 upon glasses has been employed. It therefore appears that this 



2* p. Niggli, Ccntr. f. Min., Geol. u. Pal., 1912, p. 331. 



» E. S. Larsen, U S Geol. Surr., Bull. 718, 1923, p. 21. See especially plate C. flg. A. 



*• Iderj, p. 47. 



