200 



STRATIFIED OR SEDIMENTARY ROCKS. 



and some in the other locality. The figure represents the strata as con- 

 nected and traceable from one locality to the other, but the intervening 

 portions between A and B may be removed by erosion, as shown by the 

 dotted line, or covered with water. In such case, the actual overlap- 

 ping can not be observed, if it ever existed, but the comparison in other 

 respects is the same. In widely-separated localities of course the com- 

 parison can only be made by means of fossils. Thus as the examina- 

 tion of the earth's surface progresses, with every new country examined 

 some gaps are filled up, and the series becomes more perfect. Many 

 gaps still remain unfilled. The series will continue to be made more 

 perfect, and the chronology more complete, until the geological exami- 

 nation of the earth-surface is finished. 



The second object to be attained by classification is the division and 

 subdivision of the whole series into larger and smaller groups, corre- 

 sponding to the eras, periods, and epochs of time. 



The following is an outline of the classification of Dana, slightly 

 modified. Except in the uppermost part it is carried only as far as 

 periods : 



Eras. 



Ages. 



Periods. 



Epochs. 



5. Psychozoic. 



7. Age of Man. 



Human, 23 



Recent. 



4. Cenozoic. 



6. The Age of Mam- 

 mals. 



f Quaternary, 22 

 [Tertiary, 21 



i Terrace. 

 ■s Champlain. 

 ( Glacial. 

 ( Pliocene. 

 •< Miocene. 

 ( Eocene. 



3. Mesozoic. 



5. The Age of Rep- 

 tiles. 



I Cretaceous, 20 

 < Jurassic, 13 

 ( Triassic, 1 8 







Carboniferous Age. f 



4. The Age of Aero- J 



gens and Am- j 



phibians. [ 



( Permian, 17 

 •] Carboniferous, 16 

 ( Sub-carboniferous, 15 





2. Palaeozoic. 



Devonian. 

 3. The Age of Fishes. 



r Catskill, 14 

 Chemung, 13 

 ■{ Hamilton, 12 

 j Corniferous, 11 

 [Oriskany, 10 







Silurian. 

 2. The Age of Inverte- 

 brates. 



' Helderberg, 9 

 Salina, 8 

 Niagara, 7 



« Trenton, 6 

 Canadian, 5 

 Potsdam, 4 

 Acadian, 3 





1. Archaean, or 

 Eozoic. 



1. Archaean. 



j Huronian, 2 

 { Laurentian, 1 





