PRECAMBRIAN ERA

CAMBRIAN PERIOD 
Oldest time division of the Paleozoic Era, extending from 540 to 505 million years ago. The rocks that originated during this interval of geologic time make up the Cambrian System; they contain the earliest record of abundant and varied life forms. The Cambrian Period is often divided into the Early Cambrian Epoch (540 to 520 million years ago), the Middle Cambrian Epoch (520 to 512 million years ago), and the Late Cambrian Epoch (512 to 505 million years ago).

PROTEROZOIC EON
Proterozoic Eon, the younger of the two divisions of Precambrian time, extending from 2.5 billion to 540 million years ago. It is often divided into the Early Proterozoic Era (2.5 to 1.6 billion years ago), the Middle Proterozoic Era (1.6 billion to 900 million years ago), and the Late Proterozoic Era (900 to 540 million years ago). Proterozoic rocks have been identified on all the continents and often constitute important sources of metallic ores, notably of iron, gold, copper, uranium, and nickel. It is thought that the many small protocontinents that had formed during early Precambrian time coalesced into one or several large landmasses by the initial segment of the Proterozoic. Rocks of the Proterozoic contain many definite traces of primitive life-forms--e.g., the fossil remains of bacteria and blue-green algae. See Precambrian time .

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