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| This view shows the large rubble blocks that make up the talus apron around the base of Devils Tower. The tower consists of mostly a dark igneous rock called phonolite, being coarsely porphyritic (bearing crystals including both mafic and feldspar minerals) in a fine (aphanitic) olive-gray groundmass. Much of it has a welded breccia texture (Karner and Halvorson, 1987). |
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