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._] Black dikes Even the most casual visitor to the Teton Range notices the remarkable black band that extends down the east face of Mount Moran (figs. 27 and 28) from the summit and disappears into the trees north of Leigh Lake. This is the outcropping edge of a steeply inclined dike composed of _diabase_, a nearly black igneous rock very similar to basalt. Thinner diabase dikes are visible on the east face of Middle Teton, on the south side of the Grand Teton, and in several other places in the range (see geologic map inside back cover). [Illustration: Figure 25. _Garnet crystal in pegmatite. The crystal is about 6 inches in diameter. Other minerals are feldspar (white) and clusters of white mica flakes. The mica crystals appear dark in the photograph because they are wet._] The diabase is a heavy dark-greenish-gray to black rock that turns rust brown on faces that have been exposed to the weather. It is studded with small lath-shaped crystals of feldspar that are greenish gray in the fresh rock and milk white on weathered surfaces.