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Thesis (M.S.)—University of Nebraska—Lincoln, 1962. Department of Geology.


Copyright 1962, the author. Used by permission.


The Spearfish Canyon area, in the Black Hills south of Spearfish, South Dakota, is intruded in several places by Tertiary dikes, sills, and laccoliths.These rocks are predominately porphyritic phonolites and usually consist of a lath-like orthoclase groundmass with euhedral orthoclase and pyroxene phenocrysts.Anhedral feldspathoid crystals and pyroxene microlites (usually microlites) are invariably present in the groundmass.There is a trend for all mafic minerals to be sodic.The sanidine phenocrysts are frequently zoned and sometimes perthitic, and, along with the ground mass orthoclase, are significantly altered to sericite.The feldspathoids nepheline and sodalite comprise up to 35% of the rock.There is no plagioclase except in rare perthitic phenocrysts. Ilmenite, analcite, calcite, and other accessory minerals are sometimes present, as are microscopic secondary hematite nodules.

A difference in specific gravity of specimens collected from different levels within the intrusion suggests magmatic differentiation by gravity settling.In theory, when the vertical locations of the specimens are plotted against their specific gravities, an S-shaped curve should result with the end points having approximately the same specific gravity.By analyzing the curves obtained for the intrusions in Spearfish Canyon, the relationship of the exposed outcrop to the total intrusion can be estimated.

Advisor: A. Lugn