The Hadean, the Archean, and the Proterozoic eons are commonly referred to as the Precambrian.
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An unconformity is a break in the rock record during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed.
Most layers of sediment are deposited in a horizontal position.
The Mesozoic era is the age of "middle life" on Earth
When magma works its way into a rock and crystallizes, we can assume that the intrusion is younger than the rock that has been intruded.