THE CAMBRIAN PERIOD. 



223 



on their base. It is manifest that the unconformity between formations Al and 

 2 (Fig. 93) must represent a longer interval of time between B and A than be- 

 tween B and C, since the formation Al had been longer exposed in the former 

 region. From J. to C the section records a "lost" interval, but the deposits made 

 during this interval (1 , Fig. 93) are preserved to the right of C, though they may 

 be inaccessible. 



Fig. 91. — Figure showing a series of sedimentary rocks, Al, resting on and against 



older rock, JR. 



In the case under consideration, Al, deposited before the uplift, stands for 

 the whole of the Proterozoic (Algonkian). As a matter of fact, the structure 



X - / 



/ 

 \ 



/ 



N fit 



<St?cz L&ve/ ~C\ 



/ ^ 



/ 



v • v _ x • \ 



Fig. 92. — This figure represents the region shown in Fig. 91 as having risen, or the 

 sea-level as having been lowered. Erosion has affected the upper portion of the 

 formations represented by Al. Meantime deposition of the eroded material has 

 taken place beneath the sea-level. These new deposits are represented by 1 in 

 the figure. 



of the Proterozoic is much more complex than these figures show. (See Fig. 89.) 

 Formation 2 (Fig. 93) is Cambrian. Formation 1, being inaccessible, is not 



" v ->-^ A 







<Se& Le]/e/ 



ft. 





\ 



~ \ 



X 





^^T^, 



Fig. 93. — This figure represents the same region as Figs. 91 and 92 after the land 

 has sunk so as to allow the sea to regain approximately the position which it 

 occupied in Fig. 91. Younger sediments, 2, have buried the older, 1. The forma- 

 tion represented by 2 is unconformable on Al and on JR but comformable on 1. 



classified. Were it accessible, its classification as Cambrian or as Proterozoic 

 would probably depend on its fossils. Looked at from the physical point of 



