CAMBRIAN PERIOD 



Middle Cambrian. Strata of the Middle (Acadian) Cambrian 

 are found above those of the Lower, and in addition in Texas, 

 Oklahoma, Arizona, parts of Montana, and perhaps elsewhere. 

 Since the Middle Cambrian beds contain marine fossils, their dis- 

 tribution indicates that the continent was being invaded by the sea 

 from the south and west before the close of the Middle Cambrian 

 epoch. Middle Cambrian beds are absent from much of the inte- 

 rior, if present identifications are correct. Where the Middle Cam- 

 brian rests on the Lower, the two are generally conformable. Where 

 the Middle overlaps the Lower, it is unconformable on older forma- 

 tions. 



Upper Cambrian. In the Later Cambrian (Potsdam, Saratogan, 

 or St. Croixan) epoch, the sea overspread much more of the continent, 

 for the Potsdam series covers not only the eastern and western bor- 

 ders of the continent, but much of the interior as well. The Upper 

 Cambrian is, as a rule, conformable on the Middle in the east and 

 west, but in the interior it is unconformable on pre-Cambrian 

 formations. Fig. 310 shows something of the distribution of the 

 Cambrian system as a whole. 



Basis for the Subdivisions 



We have now to inquire how the Cambrian system may be 

 recognized, and further, the means by which the Lower, Middle, 

 and Upper parts may be distinguished from one another. 



Superposition. Where a formation or series is conformable on 

 another of known age, as the Middle Cambrian on the Lower, the 

 presumption is that the upper was formed immediately after the 

 lower. In this case, the approximate age of the upper is known. 

 But where one formation is unconformable on another of known 

 age, the stratigraphic relations between them do not show whether 

 the upper is much or little younger than the lower. 



Fossils. The Cambrian is the oldest system of rocks known to 

 contain abundant fossils. Most of them represent the shells, other 

 hard parts, or tracks of marine animals buried in the sands and muds 

 when they were deposited. The fossils of any division of the 

 Cambrian system constitute the known fauna of that stage, but it 

 is not supposed that fossils of all species that lived have been 

 preserved. 



The Lower Cambrian formations contain certain fossils which 

 are distinctive. Among them are species of a genus of trilobites 



