Megascopic cristobalite has been found at several localities in the United States. Rogers has described specimens from three California localities and in his 1922 paper reports that he has obtained specimens from five others. Shannon found cristobalite crystals 0.5 mm. in size in vesicular cavities in basalt at Spokane, Washington. Foshag has described megascopic cristobalite from Obsidian Cliff in Yellowstone Park, and Larsen has reported cristobalite of visible size in southwestern Colorado. In spite of these occurrences, megascopic cristobalite is rare although no longer an oddity. Microscopic cristobalite, on the other hand, is much more widespread. Larsen (op. cit., 682) reports that it is fairly widespread in about one-tenth of the lavas of the San Juan region, but adds that it rarely makes up more than a small fraction of one per cent of any lava.