Era
Cretaceous
92
1876-1950
75
20th century
56
Tertiary
47
Precambrian
39
19th century
38
Eocene
30
1951-
29
Jurassic
29
Pennsylvanian
25
21st century
20
Paleozoic
20
To 1876
18
Civil War, 1861-1865
17
Permian
16
Triassic
16
Quaternary
15
Cenozoic
13
Oligocene
13
1830-1899
8
1956-
6
Devonian
6
Holocene
6
Mesozoic
6
Pleistocene
6
1817-1879
5
1840-1931
5
1842-1907
5
1929
5
Miocene
4
1843-1942
3
1871-1945
3
1981-1999
3
Mississippian
3
-1914
2
1794-1860
2
1809-1865
2
1812-1901
2
1823-1890
2
1838-1937
2
1839-1916
2
1848-1902
2
1851-1926
2
1860-1898
2
1861-1886
2
1864-1939
2
1866-1948
2
1867-1932
2
1869-1934
2
1871-1941
2
more ...