Years ago people disputed as to the way in which the earth was made. Those who lived where all the rocks had, like lava, the appearance of having once been melted, believed that fire had done all the work. Those who lived where the rocks appeared to be formed of hardened mud, sand, and lime, substances such as we find accumulating under water, said that water alone had been the means. But in later years the earth's surface has been more widely explored, and now it is known that both opinions were partly right. Water and fire have both been concerned in the making of the earth. In the great valleys fire-formed rocks are rare, but they are more or less abundant in all mountainous regions, for where mountains are, there the crust of the earth is weakest. There are many reasons for believing that the interior of the earth is very hot. We know that the surface is settling in some places and rising in others, and that where the strain of the upheaval is too great the rocks are broken. These convulsions sometimes cause earthquakes and sometimes volcanic eruptions, when enormous quantities of molten rock are poured out over the surface. In all the long history of our earth probably no greater flood of lava than that which made the Columbia plateau was ever spread over the surface of any region. Travel where you will over the plains of southern Idaho, central Washington, or Oregon, and examine the rocks which here and there rise above the soil or are exposed in the canons, and you will find that they all appear to have been formed by fire. [Illustration: FIG. 7.--SNAKE RIVER AT IDAHO FALLS Just beginning to cut a canon in the volcanic plateau] These rocks are dark in color and very hard. They are not arranged in regular layers like sandstone and shale; many of them show numerous little cavities which once contained steam. These cavities give to the rock a slag-like appearance. In this kind of rock, which we shall call lava, there are, of course, no remains of shells or bones of animals such as are often found in rocks formed from sand or clay. Do not picture to yourself the Columbia plateau as one continuous stretch of level land, for it is broken by many mountain ranges. Some of these are old mountains which were too tall to be buried by the lava, but most of them have been formed out of the plateau itself. The eruptions which made the plateau extended through a very long time, perhaps hundreds of thousands of years, and the older lava is deeply decayed and covered with soil. Some of the later flows show extremely rough and rugged surfaces and are probably only a few hundred years old. [Illustration: MAP OF THE COLUMBIA PLATEAU]