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Field Division of Education
The Geology of Devils Tower National Monument
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The igneous rock forming the main portion of the tower was identified by Pirsson (1894) as a phonolite rich in soda feldspar. A petrographic study was made of this rock by Albert Johannsen, a description of it appearing in Darton and O'Hara's work in the Devils Tower Folio. Johannsen refers to this rock as phonolite porphyry. "Megascopically the rock is coarsely porphyritic and is medium gray in color. It consists of large, white or colorless, short thick crystals of feldspar and minute, dark-green pyraxenes, in a gray, aphonitic groundmass. The feldspar crystals vary from one-eighth to one half inch in diameter although most are less than one quarter inch."

The microscope reveals the phenocrists to be feldspar, pyroxene, apatite, and very little magnetite and titanite. No nephalite phenocrists are found. The feldspar, a soda rich orthoclose, is in general fairly fresh, although somewhat altered along cleavage cracks to kaolin and isotropic zeolites. The pyroxene, varying from perhaps one-quarter to one-half as much in amount as the feldspar, occurs in short, thick crystals, is very fresh, and almost invariably shows zonal structure. It is green in color and as a rule darkest around the edges. The centers are augite and the outer zones are aegirite. The apatite phenocrysts are short, thick, colorless prisms or hexagonal basal sections. Magnetite occurs in a few irregular grains; and titanite, in rectangular or lozenze-shaped crystals, most of which are twinned, is rare. Apatite is present in small amounts as an accessory.

The groundmass consists of small orthoclase laths in sub-parallel position and shreds and needles of aegirite, with small clouded crystals and anhedrons between. These clouded spaces are for the most part indeterminate; some of them are clear and colorless and definitely alpali feldspar, but about half of them are brownish and clouded. Scattered through the groundmass, in some of the rock, is magnetite in exceedingly small cubes. There is much secondary material--calcite, kaolin, chlorite, analcite and ananisotropic zeolite. Very commonly a calcite center is surrounded by an analcite rim, the whole being a cavity filling. Some of these secondary products have definitely been derived from the feldspars.

Analysis of Phonolite Porphyry From Devils Tower

BaO     .05


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