SIMlKHll.lTKS AND LIT IIOPIIYS.E 21<J 



iires 1 and 8), we iiiid tlie original radial splierulitic nialeiial covered 

 witli a crust of geode-like crystals measuring up to 0.2 mm. in diameter. 

 These crystals decrease in size as we pass from the center toward the 

 margin, where they are only a few hundredths of a millimeter in diameter. 

 The majority of these lithophysal crystals are tridymite; they form 

 clusters and rosettes similar in every respect to the tridymites of specimen 

 88432. The tridymite occurs in characteristic hexagonal plates, showing 

 weak, irregular hirefringence and low refractive index, n <_ 1.480. No 

 (luaitz or cristobalite was observed with certainty. Extending into the 

 liidvmite crystals and holding them together is a radial, weakly bire- 

 fracting mineral of much higlier refractive index. Tt is probably identi- 

 cal with the fibrous, feldspar-like mineral which occurs in larger indi- 

 \ idiials in the cavities of specimen 88432, described above. 



Scattered through the mass of tridymite crystals are small (0.05 to 0.1 

 111 111. in diameter), sharply developed crystals of a honey yellow to yellow- 

 liiown mineral of the following optical properties: Refractive index a. 

 considerably higher than 1.T8, but noticeably less than 1.85T; y, slightly 

 greater than 1.85T. Optical character negative; birefringence strong: 

 crvstal system apparently orthorhombic. On a section practically iioniuil 

 to the obtuse bisectrix the angle between the crystal edges was found l)y 

 measurement to be about 100°. All of these properties agree witli those 

 of fayalite, which also occurs in similar association in the Yellowstoiie 

 Park litho-^hysae. The angle noted above is probably that l)etween (021) 

 and (021), which is 99° 06' for fayalite. 



The relative abundance of the fayalite is remarkable when we recall 

 that the total iron oxide content in the rock is less than 3 per cent. 

 Similar relations were recorded by Iddings for the Yellowstone litho- 

 physse. The fayalite crystals decrease also in size from the center toward 

 the margin of the lithophysal cube faces. The presence of fayalite and 

 tridymite as the chief minerals formed indicates precipitation from a 

 physico-chemical system different from that of the noimal rhyolite 

 magnui : the system consisted largely, of course, of volatile coinponeiiis, 

 which at the high temperatures attacked the crystals which had crystal- 

 lized from the magma itself. 'I'hese components were first set free on 

 the crystallization of the spheriilites. which in turn, at lowei' tempeia- 

 tui'es. were attacked hy them, and changes were |ii-o<luee(l which resulted 

 in new crystal phases more stal)le under the new conditions than the 

 oiiginal crypto-crystalline sid)stances of the spherulito. 'I'liere is no evi- 

 dence that during this recrystallization there was migration of material 

 away from the cavity. The decrease in size of grain of the new crystals 

 from the center to the margin indicates again that the gases acted for a 



