AliT. 17 



MINERALS OF OBSIDIAN CLIFF FOSHAG 



9 



of single crystals in the many otlicr occurrences where twins are 

 tlie rule is noted. 



In the large, hollow spherulitic lithophysae, composed of fibrous 

 feldspar and trid3'mite the latter mineral is usually grouped in 

 spherical aggregates resembling the cristobalite pellets but with a 

 rough surface and more vitreous luster. These groups of tridymite 

 are attached to the feldspar rods and project into the cavity or into 

 the spaces between the crystals of feldspar. One gains the impres- 

 sion from an examination of this occurrence tliat the tridymite is 

 a somewhat later formation than the feldspar. In the lithophysae 

 of the lithoidite also the tridymite appears to be later than the feld- 

 spar since it forms radiating groups of plates perched upon stubby 

 feldspar crystals (see fig. 2, and pi. 4, fig. 4). In the cracks of the 

 large spheruites 

 of the Firehole 

 River obsidians, 

 the tridymite is 

 dusted over the 

 surface and is 

 undoubtedly the 

 last mineral to 

 form. Some of 

 these masses of 

 radiating r o d s 

 carry pheno- 

 crysts of quartz 

 and feldspar of 

 intertelluric ori- 

 gin which are 

 likewise dusted 

 over with tridy- 

 mite plates 

 where they pro- 

 ject into a crack. Apparently the feldspars of these bodies grew 

 rather rapidly while the tridymite formed more slowly. 



Cristobalite. — The cristobalite is found only in cavities in the 

 wholly glassy rock where it is abundantly, though not always con- 

 spicuously, associated with feldspar. It is never found with the 

 coarser feldspar crystals and is evidently confined to the upper and 

 thinner portions of the flow. It forms small white pellets smoother 

 than those of the tridymite groups, wliicli are scattered over the 

 surface or through the body of the feldspar crusts (pi. 4, figs. 2 

 and 3). Clear, honey-yellow crystals of fayalite are often asso- 

 ciated wdth it. Under the binocular microscope the cristobalite shows 

 a fine, drusy surface made up of crystals which are usually too small 

 7966S— 26 2 



Fig. 2. — Section of ring in lithophysae showing the rela- 

 tion OF THE tridymite (WHITE) TO THE FELDSPAR (STIPPLED). 



Camera lucida sketch. X40 



