The face of each wall of the fissure is often beautifully polished, as if glazed, and not unfrequently striated or scored with parallel furrows and ridges, such as would be produced by the continued rubbing together of surfaces of unequal hardness. These polished surfaces are called by miners "slickensides." It is supposed that the lines of the striae indicate the direction in which the rocks were moved. During one of the minor earthquakes in Chili, which happened about the year 1840, and was described to me by an eye-witness, the brick walls of a building were rent vertically in several places, and made to vibrate for several minutes during each shock, after which they remained uninjured, and without any opening, although the line of each crack was still visible. When all movement had ceased, there were seen on the floor of the house, at the bottom of each rent, small heaps of fine brickdust, evidently produced by trituration. [Illustration: Fig. 85. Faults. A B perpendicular, C D oblique to the horizon.] It is not uncommon to find the mass of rock, on one side of a fissure, thrown up above or down below the mass with which it was once in contact on the other side. This mode of displacement is called a shift, slip, or fault. "The miner," says Playfair, describing a fault, "is often perplexed, in his subterraneous journey, by a derangement in the strata, which changes at once all those lines and bearings which had hitherto directed his course. When his mine reaches a certain plane, which is sometimes perpendicular, as in A B, fig. 85., sometimes oblique to the horizon (as in C D, ibid.), he finds the beds of rock broken asunder, those on the one side of the plane having changed their place, by sliding in a particular direction along the face of the others. In this motion they have sometimes preserved their parallelism, as in fig. 85., so that the strata on each side of the faults A B, C D, continue parallel to one another; in other cases, the strata on each side are inclined, as in _a_, _b_, _c_, _d_ (fig. 86.), though their identity is still to be recognized by their possessing the same thickness, and the same internal characters."[62-A] [Illustration: Fig. 86. E F, fault or fissure filled with rubbish, on each side of which the shifted strata are not parallel.]