If it is not always easy to deduce the orientation of the stress that caused fracturing, it is even more difficult to be sure of the type of loading that set up the stress. Whether loading of a rock-mass consists of compression, tension, or a shearing couple, the fracture pattern within the limits of a mine might be virtually the same, although study of the pattern throughout a wider region might indicate the correct interpretation.

The motivating forces that cause fracturing may be either local or regional. Of local forces, the source most commonly invoked is move¬ment related to igneous intrusion. Cracks in an intrusive can result from contraction on cooling or from collapse on withdrawal of the magma. Fractures in the adjoining rocks may also result from these causes, as well as from forceful entry of the magma. Thus, up-punching of overlying rocks can be caused by rise of the magma in the form of stocks and necks.

Local fracturing and shattering can also result from removal of ma¬terial by solution (mineralization stoping). By the latter process, Locke explains the down-dropped plugs that form the copper-bearing "ovals" of Sonora, though some other observers have attributed them to magma movement. It is interesting and perhaps puzzling to recall that rather similar though smaller down-dropped plugs are found in the Tri-State zinc mines in the complete absence of igneous rocks.

Local forces, especially those depending on the energy of igneous intrusion, have a seductive appeal for the economic geologist. The concept of a magma entering and supplying the solutions to traverse the cracks that its own entry and cooling have created is disarmingly plaus¬ible, but it is usually too simple to fit all of the facts. Its uncritical acceptance is likely to obscure not only the origin of the fractures but also the source of the ore-solutions, thus throwing the ore-hunter off the scent and tricking him into looking for ore in just the wrong places.

For the mere fact that fractures are localized in and around an intrusive does not prove that the emplacement or its aftereffects caused the fracturing. Bodies of igneous rock and their margins are just the places where failure would be likely during regional deformation, a fact that is too often overlooked. The quartz-filled fractures in the dike at Woods Point, Victoria, (see Figure 65) have been cited repeatedly in the texts as examples of shrinkage cracks, but an analysis of the pat-tern of fractures shows clearly that they are shears and testifies to failure under regional compression. The fractures in the Cripple Creek crater had long been attributed to volcanic forces or to settling, but the more careful analysis by Loughlin and Koschmann indicates that they are due to regional compression.

In short, I believe that in more instances than are generally recognized, the fracturing forces have been regional rather than local. An off-hand statement that "the veins occupy fractures probably formed during cool¬ing of the intrusive" or "the intrusion has fractured the surrounding rocks" does more harm than good unless it is supported by evidence that the pattern of fractures actually corresponds with this mode of origin rather than with another.