THE PALEOZOIC ERA 



CHAPTER XV 

 THE CAMBRIAN PERIOD 

 FORMATIONS AND PHYSICAL HISTORY 



The crustal movements which closed the Proterozoic era con- 

 verted a large area within the limits of North America into land. 

 This is shown by the distribution of the basal strata of the Cambrian, 

 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 

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

 most of the strata are of marine origin. 



Subdivisions 



The Cambrian system is divided into three parts, the Lower, 

 the Middle, and the Upper. Georgian (Vt), Acadian, and Potsdam 

 or Saratogan (N. Y.), names of localities where the several divisions 

 of Cambrian were first differentiated in North America, are syno- 

 nyms for Lower, Middle, and Upper Cambrian respectively. The 

 name St. Croixan (Wis.-Min.) also is used for the Upper Cambrian. 



Lower Cambrian. Lower Cambrian formations are known in 

 North America only near the eastern and western borders of the 

 continent (Fig. 309). In the east, they occur in the Appalachian 

 belt and at some points farther east; in the west, they are found in 

 various states between the noth and the i2oth meridians. Both 

 east and west, the strata contain marine fossils. Those of the east 

 were accumulated in straits, sounds, etc., rather than on the shores 

 of the open sea. The great tract between the Appalachian Moun- 

 tains on the one hand, and western Montana and Utah on the 

 other, is believed to have been land during the early part of the 

 period, and from it sediments were probably carried to the sea on 

 either hand. 



344 



