Untitled clearly worked out. If the successive strata contained good samples of all the plants and animals living at the time when the beds were formed, then it would be easy to read the record of the rocks, but many animals were too soft to become satisfactory fossils, many were eaten or dissolved away, many were destroyed by heat and pressure, so that the rock record is like a library very much damaged by fire and looting and decay. § 2 The Geological Time-table The long history of the earth and its inhabitants is conveniently divided into eras. Thus, just as we speak of the ancient, mediAval, and modern history of mankind, so we may speak of PalAozoic, Mesozoic and Cenozoic eras in the history of the earth as a whole. Geologists cannot tell us except in an approximate way how long the process of evolution has taken. One of the methods is to estimate how long has been required for the accumulation of the salts of the sea, for all these have been dissolved out of the rocks since rain began to
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