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Silurian period

(Encyclopedia)Silurian period sĭlo͝orˈēən, sī– [key] [from the Silures, ancient tribe of S Wales, where the period was first studied; named by the British geologist R. I. Murchison], third period of the Pal...

Ordovician period

(Encyclopedia)Ordovician period ôrdəvĭshˈən [key] [from the Ordovices, ancient tribe of N Wales], second period of the Paleozoic era of geologic time (see Geologic Timescale, tablegeologic timescale, table) fr...

Permian period

(Encyclopedia)Permian period pûrˈmēən [key] [from Perm, Russia], sixth and last period of the Paleozoic era (see Geologic Timescale, tablegeologic timescale, table) from 250 to 290 million years ago. Many ma...

Cambrian period

(Encyclopedia)Cambrian period [Lat. Cambria=Wales], first period of the Paleozoic geologic era (see Geologic Timescale, tablegeologic timescale, table) extending from approximately 570 to 505 million years ago. It ...

sidereal period

(Encyclopedia)sidereal period, in astronomy, length of time a body takes to complete an orbit relative to the fixed stars. See sidereal time. ...

Tertiary period

(Encyclopedia)Tertiary period tûrˈshēĕrˌē [key], name for the major portion of the Cenozoic era, the most recent of the geologic eras (see Geologic Timescale, tablegeologic timescale, table) from around 26 to...

Quaternary period

(Encyclopedia)Quaternary period kwətûrˈnərē [key], younger of the two geologic periods of the Cenozoic era of geologic time (see Geologic Timescale, tablegeologic timescale, table) from 2 millon years ago to t...

Triassic period

(Encyclopedia)Triassic period trīăsˈĭk [key], first period of the Mesozoic era of geologic time (see Geologic Timescale, tablegeologic timescale, table) from 205 to 250 million years ago. Throughout the Triassi...

Cretaceous period

(Encyclopedia)Cretaceous period krĭtāˈshəs [key], third and last period of the Mesozoic era of geologic time (see Geologic Timescale, tablegeologic timescale, table), lasting from approximately 144 to 65 millio...

period, in grammar

(Encyclopedia)period: see punctuation.