12 PROCEEDINGS OP THE NATIONAL MUSEUM vol. 68 



Fayalite is also present in the lithophysae of the lithoidite where 

 the crystals are somewhat more prismatic in habit and are usually 

 wholly or in part altered to hematite. In all cases it forms larger 

 crystals than any of the associated minerals. 



Quartz. — Quartz is not uncommon as phenocrysts in some of the 

 Yellowstone rhyolites but is far less abundant than feldspar. In the 

 lavas of Obsidian Cliff, however, it is absent except for minor 

 amounts in some of the lithophysae of the lithoidite where it oc- 

 curs as small prismatic crystals. In one specimen a cavity carried 

 small crystals of quartz "with small plates of tridymite dusted over- 

 them. These quartz crystals are trapezohedral in type and hence 

 apparently of the low temperature form. 



DISTRIBUTION OF THE MINERALS 



The relation of the minerals to their position in the flow was not 

 studied in the field, but certain associations suggest a vertical dis- 

 tribution. Iddings describes the surface of the flow as pumiceous, 

 passing in depth into a glass rich in cavities but barren of minerals. 

 Below this is a zone of glass carrying simple lithophysae made up 

 of fibrous feldspar and cristobalite. As more stony matter is en- 

 countered, corresponding to greater depths, the lithophysae become 

 larger, the feldspar crystals stouter, and the cristobalite is replaced 

 by tridymite. This phase passes over to the lithoidite (represent- 

 ing a still deeper zone) in which the feldspar occurs in well-defined 

 crystals and the tridymite as large, single individuals. Presumably 

 the quartz-bearing lithophysae are from the lowest or thickest parts 

 of the flow, but hand specimens do not show any direct evidence of 

 this. Fayalite is present in all zones; in the upper it is clear yellow 

 to brown and is fresh, while in the lower it is wholly or in part 

 altered to hematite. 



The type of silica mineral to form seems, therefore, to be depend- 

 ent upon the position within the flow. Since the comparatively 

 slight increase in pressure in such a flow would produce but a 

 negligible effect upon the silica inversions involved, and since the 

 temperature of consolidation in different parts of the flow was 

 essentially the same, some other factor has determined which form 

 shall be produced. As we are dealing with the metastable forms, 

 the time factor may have been an important one. 



TEMPERATURE OF FORMATION OF THE MINERALS 



In a specimen of the hyaline obsidian carrying numerous lith- 

 ophysal cavities (No. 2235 of the Iddings Collection), a small tongue 

 of glass several millimeters long and shaped like a fish-hook projects 

 through the crust into the cavity. This tongue is similar to those 



