THE PALEOZOIC ERA 



CHAPTER XVI 

 THE CAMBRIAN PERIOD 



FORMATIONS AND PHYSICAL HISTORY 



The great crustal movements which brought the Proterozoic 

 era to a close converted a large area within the limits of the North 

 American continent into land. This is shown by the geographic 

 distribution of the basal strata of the Cambrian, 1 the oldest system 

 of the Paleozoic era. Where accessible, the base of the system is, 

 in most places, unconformable on underlying formations. The 

 distribution of the successive parts of the system gives some idea 

 of the relations of sea and land throughout the period, for most of 

 the strata are of marine origin, as their fossils show. 



The Subdivisions of the, Cambrian and their Distribution 

 The Cambrian system has been divided into three series, known 

 as the Lower, Middle, and Upper Cambrian, respectively. Other 

 names are sometimes assigned to these divisions. Thus Georgian 

 (Vt.), Acadian, and Potsdam (Saratogan) (N.Y.), names of localities 

 where the corresponding divisions of Cambrian were first differen- 

 tiated in North America, are synonyms (in America) for Lower, 

 Middle, and Upper Cambrian respectively. 



The Lower Cambrian. The Lower Cambrian formations are 

 known in North America only near the eastern and western 

 borders of the continent (Fig. 357). In the east, they are found 

 in the Appalachian belt and at some points farther east; in the 

 west, they are found in various states between the 110th and the 

 120th meridians. In both the east and the west, the strata con- 

 tain marine fossils. The strata of the east were accumulated in 

 straits, sounds, etc., rather than on the shores of the open sea. 



1 A summary of the literature on the North American Cambrian prior to 

 1892 is given by Walcott in Bull. 81, U. S. Geol. Surv. 



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