Tangent diagrams are polar coordinate graphs on which the attitude of planes and lines is represented by the end point of vectors, proportional in length to the tangent of the angle of dip. They provide convenient and easily visualized vectorial solutions for such problems as finding apparent dip from true dip, true dip from two apparent dips, and the line of intersection of two planes. In addition, they have proved to be especially useful for orienting cylindrical and conical folds by graphic analysis of dip data and distinguishing cylindrical folds from the two possible kinds of conical folds. Dip measurements at random locations on cylindrical folds define straight-line “statistical” patterns on tangent diagrams. Dip data for conical folds, however, define two kinds of curved lines corresponding, respectively, to the two possible kinds of conical folds. Lines concave toward the center identify conical anticlines that narrow up-plunge (or synclines that narrow downplunge) and lines concave away from the center define conical anticlines that narrow downplunge (or conical synclines that narrow up-plunge). Although the first kind of conical fold is more common than the second, only the second kind has been treated in the literature.

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