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challenge but also a profoundly temporal one. In effect, the canyon served as a kind of self-propelled time machine, a terrestrial chronometer in which each step they took would have hurled them into a deeper and more distant precinct of the past.
    Their progression into this well of time would not have been uniform. Depending on where they were, they might have passed through fifteen millionyears in a single stride, while the next dozen paces may only have taken them through a few millennia. But regardless, every step of their journey would have catapulted them backward through entire ages and epochs as they dropped deeper and deeper into a world whose buttresses had been laid down long before anything human had ever taken place. And as they penetrated each stratum—through the hardened deltas of dead rivers, the floors of vanished oceans, the petrified dunes of antediluvian deserts—they could have calibrated their progress by noting each successive benchmark in the fossil record that they passed.
    Past the conifers, the reptiles, and the amphibians. Past the first seed plants, the first beetles, the first sharks. Past the spiders, the scorpions, and the centipedes, then past the first creatures ever to have left the ocean and ventured forth upon the land. Not long after that, they would pass the point where the first terrestrial plants—the mosses and worts and ferns—had begun to colonize the land. By this point, they’d be inside a realm that had been framed when the land was so empty and barren that all of its rivers ran over bare rock, unadorned by so much as a single green leaf.
    When you get down that deep, the rock record has big gaps, so without even realizing it, the Spaniards would have found themselves skipping over some important opening movements in the pageant of biology: the first armor-plated fishes, the first vertebrates, the first coral reefs. At around this point, too, things would begin to grow quiet as orders and classes and, eventually, entire phyla in the taxonomic regnum of life successively winked out.
    First to go would be the gastropods, followed swiftly by the sponges and the echinoderms. And by the time Cárdenas and his men had reached the layer of smooth, tan-colored sandstone now known as the Tapeats, the sea lilies and the brachiopods would also be gone, followed soon thereafter by trilobites—the horseshoe-crab-shaped creatures that had peered through the shallows of lost Paleozoic oceans with eyes made of calcite, the earliest vision systems on earth and a lyrical merger of biology and geology, a living form of rock.
    Somewhere below the last of the trilobites, Cárdenas’s company would have stepped across a final threshold and entered into the stillest and most silent epoch of all—the time of everything that preceded visible, multicelled life. This was a world that had been populated by whorled chains of the earliest cyanobacteria, anaerobic creatures whose chemistry had coalesced shortly after the crust of the planet had begun to cool and life’s initial moments of respiration unfolded amid an atmosphere devoid of a single molecule of oxygen.
    Eventually, they would have been able to go no farther. By this point, they’d be standing at the edge of the river itself, a kingdom walled off by elegant foliationsof Vishnu schist, rock that had been compressed and deformed by heat and pressure so intense that the minerals inside the stone had recrystallized and metamorphosed into something surreal and otherworldly. This was stone whose bloodlines extended further back than the human mind could possibly conceive—seventeen million centuries into the past, nearly half the life span of the planet and one-tenth the age of the universe itself. A stone so dense and so black that a man felt, upon seeing it for the first time, that its polished surface must surely mark some kind of nadir. Certainly no other rock on the surface of the earth seemed to glitter so darkly with the dawn light of