(Encyclopedia) Proterozoic eon: see Precambrian. (Encyclopedia) Black Canyon of the Gunnison National Park, 30,300 acres (12,272 hectares), W central Colorado, in the Rocky Mts. The park embraces the most spectacular stretch of the 53-mi (85-km)? (Encyclopedia) snowball earth, hypothesized condition in which the earth was covered completely by ice during several periods in the Proterozoic. The hypothesis was originally developed to explain? (Encyclopedia) schist sh?st, metamorphic rock having a foliated, or plated, structure called schistosity in which the component flaky minerals are visible to the naked eye. Schists are distinguished? (Encyclopedia) Precambrian, name of a major division of geologic time (see Geologic Timescale, table), from c.5 billion to 570 million years ago. It is now usually divided into the Archean and? It is generally assumed that planets are formed by the accretion of gas and dust in a cosmic cloud, but there is no way of estimating the length of this process. Our Earth acquired its present size?