In the eastern districts the materials denuded from the mountains and plateaus have been carried to the sea, but in the western districts the materials carried from the mountains are deposited in the adjacent valleys, so that while the mountains are composed of rocks of great age, the rocks of the valleys are of recent origin. In that geological era known as the Glacial epoch the waters of a great lake spread over these valleys, and the mountains stood as islands in the midst of a fresh-water sea. For the history of this lake we are indebted to the researches of Mr. Gilbert. It had its outlet to the north by way of the Shoshoni River and the Columbia to the North Pacific. These later beds of the valleys are in part the sediments of Lake Bonneville, the great lake above mentioned, and in part they are subaërial gravels and sands. * * * * * The Wasatch system of mountains is composed of abrupt ranges crowned with sharp peaks. The several minor ranges and groups of peaks into which it is broken are separated only in part by structural differences, since ridges with homogeneous structure are severed by transverse valleys. The drainage of the whole area occupied by the Wasatch Mountains is westward to the Great Salt Lake. The streams that head in the western end of the Uinta Mountains and West Tavaputs Plateau cut through the Wasatch Mountains. Great Salt Lake and its upper tributary, Utah Lake, exist by virtue of the presence of the Wasatch Mountains, for the mountains wring from the clouds the waters with which the lakes are supplied. Walled by high ridges and peaks, many elevated valleys are found. In the midsummer months these valleys are favored with a pleasant, invigorating climate. Occasionally showers of rain fall. Vegetation is vigorous. The distant mountain slopes bear forests of spruce, pine, and fir; the broken foot hills are often covered by low, ragged piñon pines and cedars; and the flood plains of the streams are natural meadows. About the springs and streamlets groves of aspen stand, and the streams are bordered with willows, box elders, and cottonwoods. Now and then a midsummer storm comes, bringing hail, and even snow. When the short summer ends, the aspen and box elder foliage turns to gold flecked with scarlet; the willows to crimson and russet; the meadows are quickly sered, and soon the autumn verdure presents only the somber tints of the evergreens; early snows fall, and the whole land is soon covered with a white mantle, except that here and there bleak hills and rugged peaks are swept bare by the winds. The brief, beautiful summer is followed by a long, dreary winter, and during this winter of snowfall are accumulated the waters that are to be used in fertilizing the valleys away below in the border region between the mountains and the desert basins.