Page:Popular Science Monthly Volume 69.djvu/294

290 they signify but little. They mean simply that loose soil has been shaken down. They do not go down into the underlying rock. From the true earthquake crack they may usually be known at once, because their course is determined by the topography. They are not straight. The true earthquake rift moves on in straight lines, broadly speaking, careless of topography. But topography is not careless of the earthquake rift. On either side of it, for perhaps hundreds of feet, the

rocks are crushed to flinders by the impact and grinding of the opposed Avails. An old fault is therefore marked by an excess of erosion. A valley or saddle marks its general course. Streams choose it for their basins, and when it crosses a mountain the softened rock yields to form a saddle or other form of depression. For these reasons, an earthquake fault is often marked in California by successions of dairies and of reservoirs. The valleys thus formed are fertile and well watered. For the most part, in much-faulted regions, such as form the rim of