SOUTH DAKOTA SCHOOL OF MINES 31 



CLASSIFICATION AND COKRELATION OF THE 



DEPOSITS 



The history of the earth since the advent of life on its 

 surface is commonly divided into certain time-divisions 

 called eras. Beginning with the oldest, these are the 

 Archeozoic, the Proterozoic, the Paleozoic, the Mesozoic, and 

 the Cenozoic* Each of these eras is divided into shorter 

 time-divisions known as periods, varying somewhat among 

 authors. For example the Paleozoic may be divided into the 

 Cambrian, Ordovician, Silurian, Devonian, Mississipian, 

 Pennsylvanian, and Permian periods; the Mesozoic into 

 Triassic, Jurassic and Cretaceous; the Cenozoic into the 

 Tertiary and Quaternary. The periods may in turn be 

 divided into epochs, as for example, the Tertiary into the 

 Paleocene, the Eocene, the Oligocene, the Miocene, and the 

 Pliocene epochs; the Quaternary into the Pleistocene, or 

 Glacial epoch, and the Recent or Human epoch. The rocks 

 laid down during the various epochs or periods are spoken 

 of as being grouped into formations (not to be confused 

 with the ill-defined expressions often used for any natural 

 oddity) the name of each formation being usually derived 

 from some town, stream, tribe of people, or other feature of 

 local interest where the formation was first carefully studied 

 and described. The Black Hills and the Badlands together 

 form a nearly continuous series from very old rocks to the 

 very youngest. The following section in order of deposi- 

 tion, the oldest being at the bottom shows the various for- 

 mations of this part of the country : 



*I regret the apparent advisability of following conservative 

 classification rather than joining present events with anticipated con- 

 ditions and adding the beautifully expressive term "Psychozoic Era," 

 the Age of Man, introduced by Prof. Joseph LeConte many years ago 

 and used by him in the various editions of his elements of Geology. 



