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ON THE VOLCANIC ROCKS OF MONTANA, U.S.A. 



EXPLANATION OF PLATE XX. 



Vitreous rocks of Montana. 



Fig. 1. Black obsidian containing trichites. Yellowstone district, X 250. 



2. Red obsidian showing in thin section reddish-yellow bands of glass, 



and grains of quartz, in a nearly colourless glass. Yellowstone district. 



X 25. 



3. Black spherulitic obsidian with bands of microliths and radiately crys- 



talline spherules. Yellowstone district. X 77. Nicols at 85°. 



4. Spherulitic obsidian, consisting of dull dark-greyish and vitreous black 



bands, necked with white glassy porphyritic crystals of sanidine. The 

 dull bands are composed of radiately crystalline spherules, while the 

 vitreous bands consist of glass, black in the hand-specimen and colour- 

 less in thin section. Yellowstone district. X 25. 



5. Black porphyritic obsidian with spherulitic bands. The drawing shows 



part of a section of one of the porphyritic crystals of sanidine, which 

 is surrounded by an isolated girdle of small spherules. Yellowstone 

 district. X 18. 



6. Yellowish grey spherulite rock with small porphyritic crystals of sani- 



dine. Yellowstone. X 32. Crossed Nicols. 



7. Rhyolite with peculiar Damascene structure and grains of quartz. 



Between crossed Nicols it is seen to have a microcrystalline structure 

 throughout. Gardiner's River. X 55. 



8. "Obsidian sandstone," a finely granular blackish tuff, composed of 



angular grains of vitreous rocks of different appearance, the majority 

 being perlitic, others showing merely microlithic streaks, but all pro- 

 bably derived from approximately the same source and representing 

 the disintegration and cementing in place of vitreous rocks. The 

 drawing shows part of one of the perlitic fragments, in which, between 

 crossed Nicols, an interference-cross is seen in the centres of some of 

 the perlitic spheroids, similar to the crosses seen under similar circum- 

 stances in artificially strained or compressed glass. In the lower part 

 of the field a crystal of triclinic felspar is shown. Lower Geyser basin, 

 Yellowstone district. X 18. Nicols at 85°. 



Note.— Except when otherwise stated, the drawings have been made by ordi- 

 nary transmitted light. 



