even the very pyramids.” The 18th century saw the spread of canal building, which led to the discovery of strata correlated over great distances, and James Hutton’s recognition that unconformities between successive layers implied that deposition had been interrupted by enormously long periods of tilt and erosion.
By 1788 Hutton had formulated a theory of cyclic deposition and uplift, with the earth indefinitely old, showing “no vestige of a beginning—no prospect of an end.” Hutton considered the present to be the key to the past, with geologic processes driven by the same forces as those we can see at work today.
The first argument was completely undermined after taking into account the amount of heat generated by radioactive decay.
The second depended on highly dubious theories of formation of the earth and moon and plays relatively little role in this compilation.
One outstanding feature of this drama is the role played by those who themselves were not, or not exclusively, geologists.The third act sees the entry of a newly discovered set of physical laws—those governing radioactivity.Radioactivity offered not only a resolution to the puzzle of the earth’s energy supply but also a chronology independent of questionable geologic assumptions and a depth of time more than adequate for the processes of evolution.Nevertheless, by the late 19th century the geologists included here had reached a consensus for the age of the earth of around 100 million years.Having come that far, they were initially quite reluctant to accept a further expansion of the geologic timescale by a factor of 10 or more.