In the northeastern part of the Yellowstone a large central crater was surrounded by smaller volcanoes. You can easily trace the conformation from Mount Washburn which stood upon its southeastern rim, heaped there, doubtless, by some explosion of more than common violence. This volcanic period was of long duration, perhaps hundreds of thousands of years. In the northeastern part of the park the erosion of a hill has exposed the petrified remains of thirteen large forests in layers one on top of the other, the deep intervening spaces filled with thick deposits of ashes. Thirteen consecutive times were great forests here smothered in the products of eruption. Thirteen times did years enough elapse between eruptions for soil to make and forests to grow again, each perhaps of many generations of great trees. Yellowstone's mountains, then, are decayed volcanoes, its rock is lava, its soil is ash and disintegrated lava. The resulting outline is soft and waving, with a tendency to levels. There are no pinnacled heights, no stratified, minareted walls, no precipiced cirques and glacier-shrouded peaks. Yet glaciers visited the region. The large granite boulder brought from afar and left near the west rim of the Grand Canyon with thousands of feet of rhyolite and other products of volcanism beneath it is alone sufficient proof of that. Between the periods from volcano to glacier and from glacier to to-day, stream erosion has performed its miracles. The volcanoes have been rounded and flattened, the plateaus have been built up and levelled, and the canyons of the Yellowstone, Gibbon, and Madison Rivers have been dug. Vigorous as its landscape still remains, it has thus become the natural playground for a multitude of people unaccustomed to the rigors of a powerfully accented mountain country. The fact is that, in spite of its poverty of peaks and precipices, the Yellowstone country is one of the most varied and beautiful wildernesses in the world. Among national parks it gains rather than loses by its difference. While easily penetrated, it is wild in the extreme, hinting of the prairies in its broad opens, pasture for thousands of wild ruminants, and of the loftier mountains in its distant ranges, its isolated peaks and its groups of rugged, rolling summits. In the number, magnitude, and variety of its waters it stands quite alone. It contains no less than three watersheds of importance, those of the Yellowstone, Madison, and Snake Rivers, flowing respectively north, west, and south. The waters of the Yellowstone and Madison make it an important source of the Missouri. There are minor rivers of importance in the park and innumerable lesser streams. It is a network of waterways. Its waterfalls are many, and two of them are large and important. Its lakes are many, and several are large. Yellowstone Lake is the largest of its altitude in the world.