Unlike the basically flat and even layers that lie above it, the Vishnu Schist is dark and gnarled, like caramel sauce that congealed while it was being poured. It is the oldest exposed rock on earth, formed at the time primitive life forms began to appear on the planet. Even so, any trace of this primordial life that may have been imprinted on the rock was long ago wiped out by the transformative power of heat and pressure emanating from the teeming mantle of the earth.
Whatever type of rock the Vishnu began as, dynamic volcanic forces changed it beyond all recognition. For that reason it is classified as metamorphic rock. At times during formation, molten streams of mineral matter forced their way through the schist, creating pinkish veins. Because they originated as molten rock, they are igneous.
The Vishnu is actually the base of a mountain range that may at one time have risen higher than Rockies. All of this material was worn away, leaving no trace. There is thus a gap in the geological record of about 500 million years known as the Earlier Unconformity. We know there is a gap because in other locations in the Southwest the missing layers exist, having somehow escaped erosion.
