Unearthing the Underworld: A Natural History of Rocks

This illuminating if abstruse study by paleontologist McNamara (Dragons’ Teeth and Thunderstones) explores how sedimentary rocks form and what they can teach about the evolution of life on Earth. He explains that such rocks as sandstone, mudstone, and limestone are compacted particulate matter created from the erosion of igneous and metamorphic rocks or, sometimes, from the remains of dead organisms (coal, for instance, is made of fossilized plants). “Limestone is a reflection of the evolution of life,” McNamara contends, explaining that because the rock draws its “carbonate from carbon dioxide dissolved in water,” it provides clues about past levels of atmospheric carbon dioxide and thus the environment’s ability to sustain life. Throughout, McNamara leans heavily on specialist language (“This results in the release of intracellular bicarbonate ions which on contact with the seawater induce precipitation of minute aragonite crystals”), but readers who tough it out will appreciate the glimpses he offers into the distant past, as when he describes how a Cambrian-era limestone deposit near the Dead Sea reveals the evolutionary arrival of marine invertebrates. Though dense, this still has some gems worth digging up. Photos. (July)