Ecosystems geological periods 21

If we are to describe evolution of the geological/biological ecosystem then we must have a way of estimating the time of appearance of various physical and chemical objects, including organisms. The best method we have of dating depends on studies of minerals. The so-called geological periods (Table 1.8), have to be obtained... 

The Geological Time-Scale is hierarchical, consisting of (from smallest to largest units) ages, epochs, periods, eras and eons. Each era, lasting many tens or hundreds of millions of years, is characterized by completely different conditions and unique ecosystems. 

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