Review: May 2017

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Before he became a towering figure in biology, Charles Darwin strove for greatness in an entirely different branch of science. In anticipation of a field trip to study rock strata in Wales with a former professor—shortly after graduating from Cambridge in 1831—he wrote to a friend, “I am mad about Geology.” Later that year, when he shipped out on the Beagle to circumnavigate the globe, it was as a geologist and companion to Captain Robert FitzRoy, not as a botanist or zoologist. Though he collected specimens of plants, animals, and fossils during his five-year journey, his prime interest was the observation and mapping of significant features in the landscape of South America and the islands of the Pacific and Indian Oceans.

Darwin’s geological rambles provide the framework for this impressive addition to the huge body of literature on the man and his work. Rob Wesson, scientist emeritus at the U. S. Geological Survey, is a specialist on earthquakes. Key sections of his book focus on Darwin’s experiences in Patagonia and the temblor-tossed coast of Chile, along with Wesson’s visits to the same sites almost two centuries later. Wesson has a broad understanding of the scientific environment of the 1800’s—a deep familiarity with primary and secondary sources on Darwin and his contemporaries—and he has a master storyteller’s talent for engaging and illuminating narrative.

As is well known, Darwin was strongly influenced by Charles Lyell’s Principles of Geology, which advanced the notion that the Earth’s surface was shaped by incremental forces, such as wind and water erosion acting slowly over eons, as opposed to a prevailing view that emphasized catastrophic events, such as Noah’s flood. During the voyage of the Beagle, Darwin noted many instances of what he took to be the sands and gravels from ancient beaches at elevations hundreds of feet above sea level. How they got there was a puzzle. Observations that Darwin and others made of shoreline changes after an 1835 Chilean quake made him believe that these places had been slowly uplifted over millions of years and that earthquakes were clues to the mechanism. In other places, such as in the center of coral atolls, Earth’s surface seemed to be sinking.

Darwin’s theory, developed more fully after he returned to England, was that the interior of the Earth was molten, and that continental crust, no more than twenty miles thick, floated like “a thick sheet of ice over a frozen pool,” bulging upward or downward as the fluid beneath ebbed and flowed. His notion that Earth’s surface was mobile and ratcheted up and down by seismic forces was well founded, even revolutionary, but, as Wesson notes, Darwin was wrong in many of the details.

Crucial features Darwin observed, such as elevated “beaches,” were the result of damming of valleys by glacial action -- not crustal uplift—but the influence of global ice ages was not generally recognized until late in Darwin’s life. And where Darwin rightly recognized uplift associated with earthquakes in Chile, we now visualize not sloshing magma but colliding crustal plates, a notion that only gained acceptance a century and a half after the Beagle’s return. Even so, Wesson’s account makes it clear how brilliant Darwin was as an observer and theorizer. Given the state of contemporary knowledge, he filled those roles as much in the physical sciences as he later did in the life sciences.