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.