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earth history

Tools

A	B
relative time	sequence of events
absolute time	relies on radioisotopes
law of superposition	oldest layer at bottom
unconformity	missing time sequence
epoch	small time division
era	very large time division
periods	3 in the mesozoic
cast	replaced remains
varves	sediment showing annual cycle
half life	4.5 billion for uranium
stromatolites	blue-green algae evidence
cambrian	first vertebrates appear
devonian	age of fishes
pennsylvanian	coal formation
permian	end of paleozoic
precambrian	archean, proterozoic

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