For the past twenty or twenty-five years the colorless transparent topaz crystals from the Thomas range of Utah, with their many crystal faces and wonderfully brilliant luster, have been very familiar objects with all mineral collectors. Although this interesting occurrence has been briefly described by several writers and the crystals have been accurately measured and figured,2 but little information of a detailed nature has been furnished, as to the actual environment and as to the conditions under which they were formed. An opportunity to study the crystals and the rocks in which they occur was afforded the writer by a visit to the locality during the early part of last summer, and the observations made at that time and a more leisurely study made of the large amount of material collected furnish the excuse for this paper.

The discovery of this topaz locality was first made by Henry Engelmann, . . .

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