"The conditions under which ptygmatic veins develop occur when the country-rock--as in a granitized area--is locally less competent or more yielding than quartzofelspathic magma which may intrude it. Veins therefore which are being driven forward by pressure from behind, will buckle plastically if they encounter a resistance during injection, and a succession of such buckles will produce a typical ptygmatic vein. . . . The contortions are therefore not necessarily due to the deformation of the veins by post-injection flowage or tectonic movements of the country-rock."