The Precambrian covers nearly 90% of Earth's History.
Early geologists referred to rocks below the lowest Cambrian strata as Precambrian. No Fossils were known from these rocks (later geologist would discover Precambrian fossils) and therefore could not be correlated with other Precambrian rocks found elsewhere.
Questions asked but that could not be answered included: Were the Precambrian granites in Northern Wisconsin older or younger than Precambrian granites at the bottom of the Grand Canyon or the granites that form El Capitan and Half Dome in Yosemite Park in the Sierra Nevada? How do those granites compare to granites in Canada, Scandinavia and Australia?
Today with radiometric dating and trace element mapping and isotopic fingerprinting geologist can correlate these rocks and have divided the what was called the Precambrian into three eons: The Hadean, Achaean and Proterozoic. We will only brief examine these ancient rocks.