In his three volume book Principles of Geology, Charles Lyell (1797–1875) argued for the gradual change of the earth and its climate over very long periods of time.
In the 19th century geology was an emerging discipline, principally concerned with the earth’s history. Lyell contrasted his ‘uniformitarian’ approach with the ‘catastrophism’ of, for example, the Biblical flood.
According to Lyell’s theory, past changes on the earth are observable in the present; for example, in the landscape, as with the volcano at Etna in Sicily or in the comparison of fossil shells.
Data from rock formations and fossils were brought together in the tables in the Principles of Geology
to form geological sequences.
The idea of gradualism, however, did not go unchallenged as glacial remains showed evidence of past ice ages in Europe and America.
Acceptance of the idea of an ice age, however, raised new scientific problems: what had caused them? James Croll developed an astronomical theory in which slight changes in the earth’s orbit would change the amount of heat from the sun which would reach the earth.
In this letter from to Croll, Lyell maintains his scepticism about Croll’s explanation.