He measured them, and they were two miles thick. Just above them in time, he found coal. In red and salmon rock nearby were the small tracks and tiny bones of dinosaurs. Larger ones, too. There were beds of marine phosphate. He collected cherty black shales, pure dolomites, dark dolomites, the massive sandstones of an ocean beach. He went into blue-gray caves in beautiful marine limestone. He found mud-crack-bearing shales. He saw mounds resembling anthills, which had been built by blue-green algae. Chipping with his hammer, he bagged folded and fractured schist, amphibolite, and banded gneiss-and granite that had come welling up as magma, intruding these older rocks at a time when they were far below the earth’s surface, a time that was eventually determined by potassium-argon dating. The time was 2.5 billion years before zakelijke energie vergelijken the present. Therefore, the rock that the granite invaded was a good deal older, but it had been metamorphosed, and there was no telling how long it had existed before it was changed -how far it reached back toward the age of the oldest dated rock on earth: a number approaching four billion years. In these lithic archives-randomly assembled, subsequently arranged and filed-was a completeness in every way proportionate to the valley’s unexceedable beauty. From three thousand million years ago to the tectonically restless present, a very high percentage of the epochs in the history of the earth were represented. It was no wonder that a geologist would especially be drawn to this valley. As he moved from panorama to panorama and outcrop to outcrop -relating this rock type to that mountain, this formation to that river-David gradually began to form a tentative regional picture, and after thirty years or so had placed in zakelijke energie sequential narrative the history of the valley. When new evidence and insight came along, what had once seemed logical sometimes fell into discard. When plate tectonics arrived, he embraced its revelations, or accommodated them, but by no means readily accepted them.