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If rivers today drop a thin layer of mud each year, how long would it take to build a delta hundreds of feet thick? That simple question, asked seriously, changed how people thought about the age of the Earth. Charles Lyell asked it again and again. This post covers what his Principles of Geology argued, the evidence he used, a worked example from his own book, how he shaped Charles Darwin, where he was wrong, and what students can learn from his method.
Who was Charles Lyell?
King's College London, where he was Professor of Geology, gives a short biography. Lyell (1797-1875) was a Scottish geologist who studied at Exeter College, Oxford, and first worked as a lawyer. He published his first geology paper in 1822 and, with his eyesight worsening, gave himself wholly to geology from 1827. He was knighted in 1848, received the Royal Society's Copley Medal in 1858, and died in 1875 while editing the twelfth edition of the Principles. He is buried in Westminster Abbey.
What did Principles of Geology argue?
The book appeared in three volumes between 1830 and 1833. Its subtitle promised to explain the former changes of the Earth's surface "by reference to causes now in operation". As Gordon Chancellor's introduction on Darwin Online, Charles Lyell's Principles of Geology, explains, the idea that the present is the key to the past was not new; James Hutton had argued it. What set Lyell apart was his insistence that those causes had always acted at the same intensity as today. In an 1832 review, William Whewell coined the word "uniformitarian" for this view.
What did the Temple of Serapis show?
The frontispiece of the Principles is a Roman building at Pozzuoli, near Naples, then called the Temple of Serapis. In the 1854 edition of the Principles, Lyell wrote that the building "affords in itself alone, unequivocal evidence that the relative level of land and sea has changed twice at Puzzuoli since the Christian era", each movement exceeding twenty feet. The columns are smooth at the bottom, but higher up they are riddled with holes bored by marine molluscs. So the columns had sunk into the sea, been bored, and then risen again, without falling over. Slow, ordinary movements, not catastrophes.
Worked example: how old is the Mississippi delta?
Lyell visited New Orleans in February 1846 and used measurements made by local doctors and an engineer. In the 1854 edition he gives these figures:
Delta area: about 13,600 square miles.
Assumed thickness of mud and sand: 528 feet (a tenth of a mile).
Solid matter carried down by the river each year: 3,702,758,400 cubic feet.
Work it through:
One square mile = 5,280 × 5,280 = 27,878,400 square feet.
Area in square feet: 13,600 × 27,878,400 ≈ 3.791 × 10^11.
Volume: 3.791 × 10^11 × 528 ≈ 2.002 × 10^14 cubic feet.
Time: 2.002 × 10^14 ÷ 3.703 × 10^9 ≈ 54,000 years.
Lyell's text gives 67,000 years. With the numbers printed in that passage, the arithmetic comes to about 54,000, so either he used figures not shown there or a step went astray. Either way, his conclusion holds: tens of thousands of years. He then added that the full period, "though far exceeding, perhaps, 100,000 years, must be insignificant in a geological point of view", because the older bluffs around the valley were formed earlier still.
Common mistakes: converting square miles with 5,280 instead of 5,280 squared, and forgetting that the volume needs area times thickness in the same unit.
How did Lyell shape Darwin?
The Darwin Online introduction records that Captain Robert FitzRoy gave Darwin the first volume at the start of the Beagle voyage in December 1831. Darwin had the second sent to Montevideo in November 1832 and read the third in May 1834. Seeing land raised in steps in Patagonia, he called himself a committed "Lyellian".
In the first edition of On the Origin of Species (1859), Darwin wrote: "He who can read Sir Charles Lyell's grand work on the Principles of Geology, which the future historian will recognise as having produced a revolution in natural science, yet does not admit how incomprehensibly vast have been the past periods of time, may at once close this volume." Natural selection needed that time. Our post on Darwin and Wallace picks up the story.
Where was Lyell wrong?
According to the Darwin Online introduction, Lyell resisted the idea that life had progressed over time, and he predicted that mammal fossils would turn up in much older rocks. He also mocked claims that land in the Baltic was rising, then had to accept the evidence when he saw it himself. Insisting that all causes always acted at today's intensity also left him struggling with events that have no present-day equal.
What can a student take from his way of working?
Turn a rate into a time. Many physics and chemistry problems are this same move.
State your assumptions. Lyell said plainly that 528 feet was a conjecture and gave his reasons.
Check the arithmetic, even of famous people. Redoing a calculation is how you understand it.
Fossil collectors like Mary Anning supplied the evidence that geologists of his time argued over. When multi-step unit conversions trip you up, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Next step: redo the delta calculation in SI units (1 foot = 0.3048 m) and check that you still get about 54,000 years.
