[Illustration: A. _Network of light-colored granite dikes on the
northeast face of the West Horn on Mt. Moran. The dikes cut through
gneiss in which the layers slant steeply downward to the left. The
face is about 700 feet high. Snowfield in the foreground is at the
edge of the Falling Ice Glacier._]
[Illustration: B. _Irregular dike of granite and pegmatite cutting
through dark layered gneisses near Wilderness Falls in Waterfalls
Canyon. The cliff face is about 80 feet high. Contacts of the dike
are sharp and angular and cut across the layers in the enclosing
gneiss._]
Pegmatite _dikes_ (tabular bodies of rock that, while still molten, were
forced along fractures in older rocks) commonly cut across granite
dikes, but in many places the reverse is true. Some dikes are composed
of layers of pegmatite alternating with layers of granite (fig. 26),
showing that the pegmatite and granite are nearly contemporaneous. Prof.
Bruno Giletti and his coworkers at Brown University, using the
rubidium-strontium radioactive clock, determined that the granite and
pegmatite in the Teton Range are about 2.5 billion years old.
[Illustration: Figure 24. _Angular blocks of old streaky granite
gneiss in fine-grained granite northwest of Lake Solitude. The
difference in orientation of the streaks in the gneiss blocks
indicates that the blocks have been rotated with respect to one
another and that the fine-grained granite must therefore have been
liquid at the time of intrusion. A small light-colored dike in the
upper left-hand block of gneiss ends at the edge of the block; it
intruded the gneiss before the block was broken off and incorporated
in the granite. A small dike of pegmatite cuts diagonally through
the granite just to the left of the hammer and extends into the
blocks of gneiss at both ends. This dike was intruded after the
granite had solidified. Thus, in this one small exposure we can
recognize four ages of rocks: the streaky granite gneiss, the
light-colored dike, the fine-grained granite, and the small
pegmatite dike._]