Strain is estimated at 36 points in the profile plane of an antiform consisting of slate and quartzite. The fold was collected as a 10-cm-wide and 12-cm-high specimen in the Lower Cambrian Harkless Formation, 5 km north of Papoose Flat pluton in the Inyo Mountains, California. Transmitted x-rays permit determination of the preferred orientation of the basal planes of chlorite in the slate. From the preferred orientation, strain is calculated according to March's theory. Reversal of the estimated strain unfolds the core of the fold without need : or rigid-body rotation. A thin intercalation of slate between two quartzite beds at the periphery of the fold can also be unfolded, but only if rigid-body rotation is permitted. The rotation is probably due to flexural slip between the quartzite beds at the time of folding.

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