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The structures of many oil domes as well as those of several intrusive masses have been caused by deforming forces at least one component of which was directed vertically upward. The up-warped beds of oil domes have a counterpart in domal or anticlinal flow structures seen in intrusives. Associated with them are fracture systems which, in intrusive areas, appear in systematic angular relation to the older flow structures. One of these systems represents funnel planes, converging toward a zone below the structural apex. Measured offsets along these planes (normal faults) show that they serve to widen the expanding structures. Faults of the same nature have been recognized in oil domes of Wyoming, California, Montana, and other districts. In elongate domes transverse and lo gitudinal fracture systems can be distinguished. Under favorable circumstances, they may serve as conduits of oil or gas, on the one hand, or as planes of dike invasion and subsequent mineralization, on the other. There is reason to believe that detailed studies in fracture mechanics will permit predictions as to where maxima of distension are to be sought in domes. It may be possible, also, to determine maxima of uplift in complexly faulted areas, by means of careful observations of the details in subordinate faults, and where intrusive masses are present, they will perhaps aid and supplement the evidence by the configuration of their own structures.
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