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. Cenozoic (SEE-no-zo-ic)—“recent-life” Mesozoic (MES-o-zo-ic)—“middle-life” Paleozoic (PAY-lee-o-zo-ic)—“ancient-life” Proterozoic (PRO-ter-o-zo-ic)—“primitive-life” Archeozoic (AR-kee-o-zo-ic)—“beginning-life” Archeozoic and Proterozoic rocks are commonly grouped together and referred to as Precambrian in age. The Precambrian rocks have been greatly contorted and metamorphosed, and the record of this portion of earth history is most difficult to interpret. Precambrian time represents that portion of geologic time from the beginning of earth history until the deposition of the earliest fossiliferous Cambrian strata. If the earth is as old as is believed, Precambrian time may represent as much as 85 percent of all geologic time. The _oldest_ era is at the _bottom_ of the list because this part of geologic time transpired first and was then followed by the successively younger eras which are placed above it. Therefore, the geologic time scale is always read _from the bottom of the chart upward_. This is, of course, the order in which the various portions of geologic time occurred and during which the corresponding rocks were formed. As mentioned above, each of the eras has been divided into periods, and most of these periods derive their names from the regions in which the rocks of each were first studied. For example, the Pennsylvanian rocks of North America were first studied in the State of Pennsylvania. The Paleozoic era has been divided into seven periods of geologic time. With the oldest at the bottom of the list, these periods and the source of their names are: