In the main, natural bridges are the result of one or another of several causes. A limestone cavern may be partly destroyed by streams of water, leaving a portion of the cavern with its roof still in place; the part of the roof thus remaining becomes the arch of the bridge. A branch of the Southern Railway threads a natural tunnel near Anniston, Ala., and the tunnel is the remnant of an old limestone cavern. In other cases a natural bridge is formed when bowlders, or a mass of rock, tumbling into a deep crevice is wedged and held in place. In still other instances a layer of hard or slowly weathering rock may rest upon a layer of rock which weathers rapidly. In such cases, if the rock layers form the face of a cliff, natural bridges, caverns, and overhangs are apt to result. CHAPTER XVIII STRANGE ROCK FORMATIONS--TABLE MOUNTAIN OF CALIFORNIA There are many table mountains in different parts of the world, but the one which I am about to describe is interesting both from geological and financial stand-points. The so-called Table Mountain of California is a massive natural railway embankment or colossal Chinese wall, extending through several counties, but best studied in Tuolumne County. The mountain is forty miles long, from five hundred to eight hundred feet high, and a quarter of a mile wide at the top. For the most part the top is bare of vegetation and quite level, though slanting slightly toward the south. In places at the base of its precipitous sides, and sometimes extending part way up, pine and other trees are found growing. This gigantic wall, broken through in several places by flowing rivers, is nothing more nor less than a mighty stream of congealed basaltic lava called latite, which in prehistoric times, rushing down the western flank of the high Sierras, usurped the bed of an ancient river channel, drinking up the waters and piling up its molten mass bank high. The bed of the stream being filled with lava, its waters not flowing through the gravel, were forced to find other channels. The action of the elements during subsequent ages has worn away in great part the banks of the pliocene river and eroded in places the solid slate rocks to the depth of two thousand feet, leaving this sinuous wall as a mute witness of the mighty forces of nature. On account of the excessive hardness and durability of this kind of basalt, this monumental fortress will endure long after the corroding tooth of time shall have crumbled to dust the royal pyramids and their very memory shall have been lost in oblivion. Some geologists think there were two volcanic streams of lava, one succeeding the other by an interval of thousands of years, the first covering the auriferous gravel and the second quenching the waters of a subsequent river which had forced a passageway through the first flow of lava.