During a seismic survey in Big Horn County, Montana, a number of deep reflections from 7.0 to 8.5 seconds were observed. The reflecting surfaces are relatively flat as shown by the small step-outs. It is demonstrated that these reflections cannot be multiples and that they therefore must come from inside the basement at depth of 18 to 21 km. The nature of the reflecting surfaces cannot be determined from available data, but it is pointed out that they fall in the depth range generally stated for the depth of the granitic layer.

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