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)—“earlier-life” Archeozoic (AR-kee-o-zo-ic)—“beginning-life” Archeozoic and Proterozoic rocks are commonly grouped together and referred to as Precambrian in age. In most places 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. Precambrian time probably represents as much as 85 percent of all geologic time. The _oldest_ era is at the _bottom_ of the time scale because this part of geologic time transpired first and was then followed by the successively younger eras which are placed above it. 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. [Illustration: Fig. 7. Generalized geologic map of Palo Duro Canyon State Park.] EXPLANATION Q & T Pleistocene and Pliocene undifferentiated Rdo Dockum Group P Permian undifferentiated 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: Permian (PUR-me-un)—from the Province of Perm in Russia Pennsylvanian (pen-sil-VAIN-yun)—from the State of Pennsylvania Mississippian (miss-i-SIP-i-un)—from the Upper Mississippi Valley Devonian (de-VO-ni-un)—from Devonshire, England Silurian (si-LOO-ri-un)—for the Silures, an ancient tribe of Britain Ordovician (or-doe-VISH-un)—for the Ordovices, an ancient tribe of Britain Cambrian (KAM-bri-un)—from the Latin word _Cambria_, meaning Wales The Carboniferous Period in Europe includes the Mississippian and Pennsylvanian Periods of North America. Although this classification is no longer used in the United States, the term Carboniferous is found in many of the earlier geological publications and on many of the earlier geologic maps. The periods of the Mesozoic Era and the source of their names are: Cretaceous (cre-TAY-shus)—from the Latin word _creta_, meaning chalky Jurassic (joo-RAS-ik)—from the Jura Mountains of Europe Triassic (try-ASS-ik)—from the Latin word _triad_, meaning three