Paleozoic 22
Mesozoic 21
Precambrian 21
Cenozoic 16
Tertiary 16
20th century 10
Cretaceous 8
Miocene 8
19th century 7
Devonian 7
Pliocene 7
Mississippian 6
To 1912 6
Triassic 6
Quaternary 5
1848-1929 4
Jurassic 4
Permian 4
Pre-Quaternary 4
Proterozoic 4
1834-1902 3
1851-1887 3
1866-1895 3
Holocene 3
Pennsylvanian 3
To 1950 3
1885-1945 2
1900-1986 2
1912-1950 2
1927-1993 2
21st century 2
Cambrian 2
Carboniferous 2
Pleistocene 2
-1781 1
-2003 1
1506-1552 1
1738-1781 1
1787-1849 1
1805-1880 1
1822-1894 1
1825-1902 1
1842-1919 1
1843-1907 1
1844-1926 1
1846-1922 1
1846-1927 1
1846-1931 1
1848 or 1849-1947 1
1848- 1
1848-1950 1
1850-1937 1
1851-1882 1
1852-1933 1
1858-1936 1
1861-1947 1
1877-1972 1
1882-1959 1
1887-1979 1
1895-1983 1
1912-2005 1
1922-2007 1
1930-2023 1
1934- 1
1945- 1
1946- 1
1951- 1
1952- 1
1959- 1
1967- 1
1970- 1
1988- 1
2020 1
Archaean 1
Civil War, 1861-1865 1
Ming-Qing dynasties, 1368-1912 1
Modern period, 1500- 1
Oligocene 1
Ordovician 1
Song-Yuan dynasties, 960-1368 1
To 1500 1