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A Pre-Cambrian Gneiss from the San Rafael Swell, Utah

Robert A. Hatch
The Journal of Geology
Vol. 49, No. 6 (Aug. - Sep., 1941), pp. 657-668
Stable URL: http://www.jstor.org/stable/30079643
Page Count: 12
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
A Pre-Cambrian Gneiss from the San Rafael Swell, Utah
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Abstract

A drilling unit of the California Company penetrated pre-Cambrian gneiss in the San Rafael swell, Utah. A detailed microscopic description of the rock and its minerals is presented. The mineralogical composition and many of the textural features indicate that the gneiss had an igneous origin and should be named a "granodiorite gneiss." The foliation was probably caused by flow of the partly crystallized magma. The early separation of the plagioclase in the form of large tabular grains is an important factor contributing to the foliation and inequigranularity of the rock and accounts for the high degree of orientation of the plagioclase grains parallel to the plane of foliation. The survival of many typically igneous features is strong evidence that the gneiss was not foliated or recrystallized after its complete solidification.

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