In the Tertiary Period, which was next in order of time after the Cretaceous, changes of the greatest importance occurred, consisting principally in the outpouring of enormous masses of volcanic material. The origin of these lava flows has been traced to a few craters, one of which was near Mt. Washburn, another in the Red Mountain Range, and a third near the sources of the Lamar River. Mt. Washburn has long been recognized as part of the rim of an ancient volcano. Both it and Mt. Sheridan, the two mountains which bore the principal part in working out the present features of that country, still remain the most prominent peaks from which the modern visitor can contemplate the work they have performed. The outpourings at first consisted of andesitic lavas. They largely changed the appearance of the mountain ranges and to some extent filled up the interior basin. The flows were not continuous but were separated by long intervals of quiet, during which vegetation and the agencies of erosion were actively at work. After the cessation of the andesitic eruptions, a quiescent period of great length ensued. Then came the period of rhyolitic flows, the centers of volcanic activity being as before Mts. Washburn and Sheridan. These flows built up the present Park plateau, and constitute the great bulk of the rocks which the tourist now sees. Following the period of rhyolitic eruptions, orographic agencies were active in producing extensive faults or displacements, which in certain localities radically changed the relative positions of the rocks. The last exhibitions of volcanic energy were in the form of basaltic eruptions. These took place in part through ordinary volcanic craters, and in part through cracks or seams in the rocks, where they may still be seen forming extensive dykes. The basalt is of relatively limited extent, but its striking and picturesque forms wherever it appears make it more interesting to the tourist than any of the other rocks. The great variety of superficial appearances which these volcanic rocks have assumed makes the Park one of the best laboratories in the world for their study. The continuance of these various outpourings doubtless extended into Quaternary time. Then came the Glacial Epoch, the epoch of wide-spread ice-carving, which still further modified the surface of the country. The paths of the ancient glaciers have in several instances been made out and their transported material may readily be distinguished. One glacier flowed from the Gallatin Range eastward across Terrace Mountain, where it joined another moving westwardly from the Absaroka Range. The united streams continued down the Gardiner and Yellowstone Valleys, in which vast masses of drift still mark their ancient route.