The _geologic time scale_ (fig. 6) is composed of named intervals of geologic time during which were deposited the rocks of the geologic column. These time intervals bear the same names that are used to distinguish the various units of the geologic column. For example, one can speak of Permian _time_ (referring to the geologic time scale) or of Permian _rocks_ (referring to rock units of Permian age in the geologic column). Both the geologic column and the geologic time scale are based upon the _principle of superposition_. This basic geologic concept states that unless a series of sedimentary rock has been overturned, a given rock layer is older than the strata above it, and younger than all of the layers below it. Thus, the field relationship of the rocks plus the type of fossils (if present) give the geologist some indication of the _relative_ age of the rocks. Relative age does not imply age in years; rather, it fixes age in relation to other events that are recorded in the rocks. Within recent years, however, it has become possible to assign ages in years to certain rock units. This is accomplished by a system of rock dating based on very precise measurements of amounts of radioactive elements (such as uranium). When present in the rocks, radioactive minerals change or decay at a known rate so that they are natural “clocks.” This method of dating has made it possible to devise a time scale in years which gives some idea of the tremendous amount of time that has passed since the oldest known rocks were formed. It has also been used to verify the previously determined relative ages of the various rock units. The largest unit of geologic time is an _era_, and each era is divided into smaller time units called _periods_. A period of geologic time is divided into _epochs_, which, in turn, may be subdivided into still smaller units. The geologic time scale might be roughly compared to the calendar in which the year is divided into months, months into weeks, and weeks into days. Unlike years, however, geologic time units are arbitrary and of unequal duration, and the geologist cannot be positive about the exact length of time involved in each unit. The time scale does, however, provide a standard by which he can discuss the age of fossils and their surrounding rocks. By referring to the time scale it may be possible, for instance, to state that a certain event occurred during the Paleozoic Era in the same sense that one might say that something happened during the American Revolution. There are five eras of geologic time, and each has been given a name that is descriptive of the degree of life development that characterizes that era. Hence, Paleozoic means “ancient-life” and the era was so named because of the relatively simple and ancient stage of life development. The eras, a guide to their pronunciation, and the literal translation of each name is shown below.