482 



GEOLOGY 



strata may be recognized, and further the means by which the 

 Lower Cambrian may be distinguished from the Middle, and the 

 Middle from the Upper. 



Superposition. Where a formation is conformable on another 

 of known age, as the Middle Cambrian on the Lower, the presump- 

 tion is strong that the upper was formed after the lower, without 

 interruption. 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 

 determine the age of the upper, beyond the general fact that it is 

 younger than the lower. 



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

 contain abundant fossils. Most of them represent the shells or 

 other hard parts of marine animals buried in the sands and muds 

 at the time they were deposited. The fossils in the strata of any 

 division of the Cambrian constitute the known fauna of that stage, 

 though it is not to be supposed that fossils of all the species that 

 lived have been preserved. 



The Lower Cambrian series contains certain fossils which are 



Fig. 359. Three characteristic Cambrian trilobites: a, Olenellus gilberti 

 Meek; b, Paradoxides bohemicus Boeckjc, Dikellocephalus pcpinensis Owen. 



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

 as Olenellus (Fig. 359, a). Along with the representatives of this 



