Time Before Time

Some words used in this book, such as "Paleozoic" and "Mississippian," label fossils by one of their most important characteristics—their place in the span of time. When a fossil played its part in the parade of life is just as significant in geological history as when certain men and nations played their parts in human history. Geologic time, far longer than historic time, reaches back into the past several billion years, and it is also, of course, less definitely documented than written history.

Geologists work with two kinds of time: relative time and absolute time. The difference in the two forms is simple. Relative time places organisms or earth events in a time sequence, in an orderly series, just as we would call the roll of the rulers of England, saying that James I came after Queen Elizabeth, and Charles I after James I, etc. The Bible chronicles events in history in the same relative fashion, not by giving dates but by placing men and events in chronological order.

Absolute time, on the other hand, reckons elapsed time since a rock or fossil was formed by measuring the rate of decay of radioactive elements and expressing the change in terms of years.

70 FOSSILS FOR AMATEURS

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