THE CAMBRIAN PERIOD. 219 



would give the maximum area covered by the Cambrian seas, for the 

 Cambrian formations were, so far as known, laid down on the floor of 

 the sea. If the exact area of the sea at any stage of the Cambrian 

 period were determined, the exact area of the land at the same stage 

 would obviously be known. If these data could be determined for 

 each stage of the period, they would constitute the record of its geo- 

 graphic changes, so far as distribution of land and water is concerned. 

 These determinations cannot be made for every stage of the period, 

 but approximations may be made for certain stages, and these approxi- 

 mations give a valuable insight into the geography of the times. 



The Subdivisions of the Cambrian and their Distribution. 

 The Cambrian period has been, for convenience, divided into three 

 parts, which may be called the Earlier, the Middle, and the Later divi- 

 sions. The system of rocks deposited during the period has been divided 

 into three corresponding series, known as the Lower, Middle, and Upper 

 Cambrian respectively. Other names are sometimes assigned to these 

 divisions. Thus Georgian (Vt.), Acadian, and Potsdam (N. Y.), names 

 of localities where the corresponding divisions of Cambrian were first 

 carefully studied, are synonyms for Lower, Middle, and Upper Cambrian 

 respectively, as these terms are used in North America. Saratogan x 

 and St. Croix 2 are also used as synonyms for Potsdam. The approxi- 

 mate distribution of the beds of sediment laid down in each of the 

 three divisions of the period is known, and the variations in the dis- 

 tribution of the three series record the changes in the relations of land 

 and water during the Cambrian period. 



The Lower Cambrian. 

 Distribution. — Fig. 90 shows the known surface distribution of the 

 Lower and Middle Cambrian (Georgian and Acadian) series. The 

 black portions represent the areas where these series come to the surface. 

 The parts shaded with lines represent the areas within the limits of the 

 continent where the same series are believed to occur, though now 

 buried. The longer the lines, the better the basis for the inference 

 that one or both series are present. North of Mexico, the unshaded 

 areas shown in the figure are thought to represent approximately the 



1 Recently proposed by the U. S. Geol. Surv., 24th Ann. Rept., p. 27, to take the 

 place of Potsdam. 



2 Not much used except in Minnesota. 



