The black dikes must be the youngest of the Precambrian units because they cut across all other Precambrian rocks. The dikes must have been intruded before the beginning of Cambrian deposition inasmuch as they do not cut the oldest Cambrian beds. Gneiss adjacent to the dike on Mount Moran contains biotite that was heated and altered about 1.3 billion years ago according to Professor Giletti. The alteration is believed to have occurred when the dike was emplaced; therefore this and similar dikes elsewhere in the range are probably about 1.3 billion years old. Quartzite At about the same time as the dikes were being intruded in the Tetons, many thousands of feet of sedimentary rocks, chiefly sandstone, were deposited in western Montana, 200 miles northwest of Grand Teton National Park. The sandstone was later recrystallized and recemented and became a very dense hard rock called _quartzite_. Similar quartzite, possibly part of the same deposit, was laid down west of the north end of the Teton Range, within the area now called the Snake River downwarp (fig. 1). The visitor who hikes or camps anywhere on the floor of Jackson Hole becomes painfully aware of the thousands upon thousands of remarkably rounded hard quartzite boulders. He wonders where they came from because nowhere in the adjacent mountains is this rock type exposed. The answer is that the quartzites were derived from a long-vanished uplift (figs. 42 and 46), carried eastward by powerful rivers past the north end of the Teton Range, and then were deposited in a vast sheet of gravel that covered much of Jackson Hole 60 to 80 million years ago. Since then, these virtually indestructible boulders have been re-worked many times by streams and ice, yet still retain the characteristics of the original ancient sediments. A backward glance So far we have seen that the Precambrian basement exposed in the Teton Range contains a complex array of rocks of diverse origins and various ages. Before passing on to the younger rocks, reference to our yardstick may help to place the Precambrian events in their proper perspective. In all of Precambrian time, which encompasses more than 85 percent of the history of the earth (31 of the 36 inches of our yardstick), only two events are dated in the Teton Range: the intrusion of granite and pegmatite about 2.5 billion years ago, and the emplacement of the black dikes about 1.3 billion years ago. These dates are indicated by heavy arrows on the time scale (fig. 30). The ancient gneisses and schists were formed sometime before 2.5 billion years ago, and probably are no older than 3.5 billion years, the age of the oldest rocks dated anywhere in the world. The close of the Precambrian—end of the beginning