Probably the most reliable of these estimates was produced by the British geologist Charles Lyell, who estimated that 240 million years have passed since the appearance of the first animals with shells.Today scientists know his estimate was too young; we know that this occurred about 530 million years ago.
In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter.
This pattern of growth results in alternating bands of light-colored, low density "early wood" and dark, high density "late wood".
As we learned in the previous lesson, index fossils and superposition are effective methods of determining the relative age of objects.
In other words, you can use superposition to tell you that one rock layer is older than another.
For example, geologists measured how fast streams deposited sediment, in order to try to calculate how long the stream had been in existence.
Not surprisingly, these methods resulted in wildly different estimates, from a few million years to "quadrillions of years".
These thick layers alternate with thin, clay-rich layers deposited during the winter.
The resulting layers, called varves, give scientists clues about past climate conditions.
From these assumptions, he calculated that the Earth was 100 million years old.
This estimate was a blow to geologists and supporters of Charles Darwin's theory of evolution, which required an older Earth to provide time for evolution to take place.
While tree rings and other annual layers are useful for dating relatively recent events, they are not of much use on the vast scale of geologic time.