In general, prospecting for oil differs pretty radically from prospecting for most other minerals. A very common way of locating an ore-mine is by the nature of the out-crop,--that is, the broken edges of strata of rocks protruding from hillsides, or tilted at an angle on level areas. If the ore-bearing vein is harder than the surrounding strata it will be found as a jutting edge, protruding beyond the surface of the other layers of rocks which, being softer, are more easily worn away. On the other hand, if this stratum is soft or decomposable it will show as a depression, or "sag" as it is called. Of course such protrusions and depressions may only be seen and examined where the rocks themselves are exposed; vegetation, drift, and snow preventing such observations. But the vegetation may in itself serve as a guide to the experienced prospector in determining the location of a mine, peculiar mineral conditions being conducive to the growth of certain forms of vegetation, or to the arrangement of such growth. Alterations in the color of the rocks on a hillside are also important guides, as such discolorations frequently indicate that oxidizable minerals are located above. In hilly or mountainous regions, where the underlying rocks are covered with earth, portions of these surfaces are sometimes uncovered by the method known as "booming." In using this method the prospector selects a convenient depression near the top of a hill and builds a temporary dam across the point corresponding to the lowest outlet. When snow and rain have turned the basin so formed into a lake, the dam is burst and the water rushing down the hillside cuts away the overlying dirt, exposing the rocks beneath. This method is effective and inexpensive. The beds of streams, particularly those in hilly and mountainous regions, are fertile fields for prospecting, particularly for precious metals. Stones and pebbles found in the bed are likely to reveal the ore-foundations along the course of the stream, and the shape of these pebbles helps in determining the approximate location of such foundations. An ore-bearing pebble, well worn and rounded, has probably traveled some little distance from its original source, being rounded and worn in its passage down the stream. On the other hand, if it is still angular it has come a much shorter distance, and the prospector will be guided accordingly in his search for the ore-vein.