THE CAMBRIAN PERIOD 57 



General New York and New England 



3. Upper Cambrian Saratogan (Croixian) series J ^^^^ t 



(Dikellocephalus fauna) Potsdam sandstone. 



2. Middle Cambrian Acadian series j Stissing limestone. 



(Paradoxides fauna) i . . 



_ , . ~ . /TTT , • n • Georgia slate, quartzite 



1. Lower Cambrian .... Georgian (Waucobian) series I an ^ limest{ j n e 



(Olenellus fauna) | Poughquag qua rtzite. 



In the typical regions these strata are superposed one above 

 the other in regular order without unconformity, but careful study 

 has shown that, passing upward in the system of strata, there is 

 a gradual change in the character of the fossils, particularly the 

 Trilobites which are so common and widespread in the rocks. Thus 

 the Lower Cambrian strata are generally characterized by the 

 Trilobite genus Olenellus, with its various species, and this charac- 

 teristic assemblage of Trilobites is called the Olenellus fauna. This 

 does not mean that Olenellus invariably occurs in Lower Cambrian 

 strata, or that other genera of Trilobites and other fossils may not 

 be present. In a similar way the Paradoxides and the Dikello- 

 cephalus faunas are the chief characteristics of the Middle and 

 Upper Cambrian respectively. Such stages or life zones in the 

 geologic column are commonly referred to as horizons. It should 

 be made clear that the genus Olenellus became extinct before the 

 Middle Cambrian strata were deposited; the Paradoxides dis- 

 appeared before the Upper Cambrian was deposited; and the 

 Dikellocephalus before the deposition of the succeeding Ordovi- 

 cian strata, though it is not meant that sharp lines separate these 

 faunas. Thus each of the faunas becomes an important geologic 

 time or horizon marker. A representative of each of these genera 

 of Trilobites is shown in Fig. 39. 



These principles, here laid down as a basis for the subdivision 

 of the Cambrian system, apply equally well to the succeeding rock 

 systems, though many organisms other than Trilobites are used 

 for the purpose. 



Distribution and Character of the Rocks 



General Distribution. — On the accompanying map (Fig. 26) 

 the surface distribution of Cambrian rocks is shown, that is to say 

 the locations of the areas in which Cambrian strata are known to 

 outcrop. The principal areas are seen to be in Newfoundland, 



