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:
[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 will be 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
[In] Texas, the [Cretaceous] has two divisions, known as either Lower Cretaceous and Upper Cretaceous or as Comanche [series] and Gulf series, respectively. These designations are for rocks of nearly equivalent age, and both sets of terms have been used by geologists and in publications. In this handbook, both sets of terms are used interchangeably, that is, Lower Cretaceous and/or Comanche series and Upper Cretaceous and/or Gulf series.
The [Cenozoic] periods derived their names from an old outdated [system] of classification which divided all of the earth’s rocks into four groups. The two divisions listed below are the only names of this system which are still in use:
[Quaternary] (kwah-TUR-nuh-ri) [Tertiary] (TUR-shi-ri)