The eruptions continued, filling the valleys little by little, until in places the lava reached a thickness of nearly four thousand feet. The lower mountains were hidden from sight. We know of the existence of these buried mountains because the wearing away of the lava in some places has exposed their summits to view. The lava flood reached farther and farther. In southern Idaho it formed the Snake River plains, which must have been, when first formed, hundreds of miles long, seventy-five miles wide, and almost as even as a floor. If we could have looked on while these things were taking place it would have appeared as if the whole land was about to sink under the fiery mass which flowed out of the earth. The streams and valleys were completely buried. The region of the John Day Lake, with all its animal remains, was covered. The lava, like a sea, crept up against the mountains surrounding the plateau region. Bays of lava extended into the valleys among the mountains, while mountain ridges rose like islands and capes from the surface of the flood. We never tire of looking at the lofty snow-capped peaks of the Cascade Range. A dozen of them rise over ten thousand feet, and two, Mounts Shasta and Ranier, are more than fourteen thousand feet high. All these mountains were formed of material thrown out of the interior of the earth during the building of the Columbia plateau. The process was very similar for each. About some one exceptionally active crater immense quantities of scoriae[1] and lapilli[2] accumulated. Then came streams of fiery lava, some of which, hardening upon the outer slopes of the crater, added still more to the growth of the mountain. The process was very slow, however. A time of eruption, marked by tremblings of the earth, explosive noises, and a sky filled with dust and clouds, might last for many years. Then came a long period of rest when the falling rains, gathering in dashing torrents, cut deep gullies down the sides of the mountain. [Footnote 1: _scorioe_, cellular, slaggy lava.] [Footnote 2: _lapilli_, volcanic ashes, consisting of small, angular, stony fragments.] [Illustration: FIG. 9.--PITT RIVER CANON, NORTHERN CALIFORNIA The plateau is built of layers of lava] The volcanoes at last ceased to grow any higher, for the lava, if the eruptions continued, formed new craters at their bases. It is probable that all these great peaks have been extinct for several thousand years, although some are much older and more worn away than others. One of these volcanoes has completely disappeared, and in its place lies that wonderful sheet of water known as Crater Lake. It is thought that the interior of this mountain was melted away during a period of activity, and that the outer portion fell in, leaving a crater five miles across and nearly a mile deep.