Geological Survey Bulletin: Issue 974

1949-01 · U.S. Government Printing Office

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The origin of the gneissic rocks on the eastern border of the Idaho batholith in the Bitterroot Range, near Hamilton, Mont., has long been in dispute. Lindgren regarded these rocks as the products of stresses related to a normal fault along the front of the range with an eastward dip of about 15°. He thought both the hanging wall and the footwall had moved, with a total displacement along the fault plane of at least 20,000 feet. The faulting was believed to have been so recent as to be a major factor in the present topography. Langton appears to accept the concept of faulting but to regard the gneissic rocks as formed much earlier from a granitic rock that was more silicic and older than the Idaho batholith. Reexamination of the evidence in the field and laboratory leads to the conclusion that the gneiss bordering the batholith in the Hamilton quadrangle more nearly resembles a sedimentary than an igneous rock, both in general aspect and in details of texture and composition. It is a stratified rock that has been recrystallized but has retained even the minor features of its original lamination. Feldspars and other minerals have been developed in it, probably as a result of permeation by emanations from the batholith during intrusion. The rocks from which the present gneissic mass developed differed among themselves in their details of composition. They are thought to have been components of the Ravalli group of the Belt series, with their characteristics now obscured by partial granitization. So many features of sedimentary rocks are present that the concept of the gneiss's derivation from an old granite is not regarded as acceptable.