How does the geologist determine the actual amount of displacement, especially in the case of a large fault in stratified rocks? First, the various formations of the region, where unaffected by faulting, are carefully studied, especially in regard to the character and thickness of each, and their relative geologic ages or normal order as they were deposited one layer above the other. Then, in the simple case of a normal-fault surface at right angles to horizontal strata, it is only necessary to find out what two formations or parts of formations come together along the fault fracture, and the actual amount of displacement is readily determined. Where strata and normal fault surfaces lie at various angles, and also in thrust faults, those angles must be determined in addition to the data above named. In many mining regions, where valuable deposits are affected by faulting, accurate knowledge of the direction and amount of displacements of faults is of great economic importance. A few examples of normal faults from well-known districts will now be briefly described. The whole eastern front of the central and southern Sierra Nevada Range of California is a great, steep fault slope, from a few thousand to ten thousand or more feet high and hundreds of miles long, of such recent geologic age that it has been only moderately affected by erosion. In fact, it is well known that the southern two-thirds of the range is a great tilted fault block, the total displacement having resulted from repeated sudden movements since about the middle of the present geologic era. A great fault also extends along the eastern base of the great Wasatch Range of Utah and the steep slope thousands of feet high is a fault scarp only slightly modified by erosion. Renewed movements along this profound fault have very recently taken place as proved by the presence of fresh fault scarps in loose deposits which have accumulated across the mouths of some of the canyons, as, for example, near Ogden. In fact, practically all of the north-south ranges of the Great Basin from Utah to California are essentially a series of tilted fault blocks. Another great fault, less conspicuous from the topographic standpoint, is hundreds of miles long in the Coast Range Mountains of California. At the time of the San Francisco earthquake of 1906 there was a renewed sudden movement along this great fracture. The eastern one-half of the Adirondack Mountains of New York is literally a mosaic of hundreds of fault blocks. Many of these faults are from two to thirty miles long and they commonly show displacements of from a few hundred to 2,000 or more feet. A glance at the geological map (in colors) of the vicinity of the great copper mines at Bisbee, Arizona, shows most of that region to contain a network of normal faults which separate it into a mosaic of fault blocks. In each of the examples of faults just given a block of earth has sunk relative to the other, or in other words, each is a "normal