The summit of Sugarloaf Mountain in Maryland. An 1830 bestseller, “Principles of Geology,” argued that geologic changes on Earth happened over millions of years, challenging the idea of a young planet. (Andrea Bruce Woodall/THE WAHSHINGTON POST)

Timothy R. Smith is a former editorial aide of Book World.

In 1650, Archbishop James Ussher of Armagh counted the Bible’s begats and determined that God created the Earth on the evening preceding Oct. 23, 4004 B.C. The idea of a young Earth was accepted by clergy and academics and was often printed in the margins of Bibles.

But in 1830, Charles Lyell, a lawyer with a passion for rocks, published “Principles of Geology,” which argued that the Earth’s geologic changes took place over millions of years. An elegantly written book, it was a bestseller on par with the works of Jane Austen and Lord Byron. It went through multiple editions. A young Charles Darwin brought a copy of it with him on the HMS Beagle and read it before he reached his first destination. The idea of an ancient Earth would have a profound impact on the young man’s thinking. And the world:

“It stirred him to wonder about the changes in life forms over time,” Brenda Maddox writes in “Reading the Rocks,” her lovely, spry history of Victorian geologists. “He shifted his attention from rocks and fossils to man — a direction that would lead him to write ‘On the Origin of Species.’ ”

Maddox points out that such was Darwin’s passion for geology that his notebook contained more jottings about rocks than creatures.

“Reading the Rocks,” by Brenda Maddox (Bloomsbury)

Victorian scientists found evidence of an Ice Age and determined that the world was once dominated by monstrous reptiles, long before man traipsed on his two legs.

Rock and fossil hunting was a popular pastime among the 19th-century British public. One could often find a top-hatted gentleman and his wife exploring the shores of Dover or the Scottish highlands. There wasn’t much barrier for entry, either. All one needed, a Victorian writer noted, was “a quick eye, a good judgment, a clear notion of what had already been accomplished, and a stout pair of legs.”

“Reading the Rocks” is filled with interesting characters: The surveyor William Smith, who created the world’s first nationwide geologic map, was protected from pillaging highwaymen by a guard with a blunderbuss. The meetings of the Geological Society were so raucous they resembled the noisy proceedings of Charles Dickens’s Pickwick Club. Two scientists had a dispute so intense over the division of two geologic ages that they didn’t talk to each other for 20 years.

The fossil collector Mary Anning would scour the coastline near her home in Lyme Regis “in her long skirt, shawl, bonnet and basket.” Her discoveries include the first fossil ichthyosaur, a marine reptile; the first two complete fossil reptiles known as plesiosaurs; and the first pterosaur, a flying reptile, found outside Germany. She was the greatest fossil hunter ever known.

Later generations of scientists would uncover continental drift and radiometric dating, and would begin to understand the cataclysm that exterminated the dinosaurs, but the Victorians launched the first great broadside against young-Earth creationism. Their discoveries, which culminated in Darwin’s theory of evolution by natural selection, “led them,” Maddox notes, “to a bigger truth than they had been looking for.”

Reading the Rocks
How Victorian Geologists Discovered the Secret of Life

By Brenda Maddox

Bloomsbury. 254 pp. $28