THE PROTEROZOIC ERA 



449 



degraded; and third, another change resulting in the deposition 

 of the upper series on the eroded surface (Figs. 305 to 307) . 



A sequence of events which might have given rise to the uncon- 

 formable relations of the Archean and the Proterozoic, where seen, 

 is illustrated by Figs. 342 and 343. Fig. 342 represents an area 

 of land composed of Archean rock in such a position as to suffer 

 erosion. The sediments derived from it are washed down to the 





Fig. 342. Diagram showing Archean land (JK) with sedimentation, a, 

 along its borders. The coarser sediments are being deposited near the 

 shore; the finer, farther from it. (Compare Fig. 343.) 



Fig. 343. Diagram representing the same region as Fig. 342, after sub- 

 sidence. The a of this figure corresponds to a of Fig. 342. It is to be 

 noted that fine sediments overlie the coarse sediments deposited at an 

 earlier time in the right-hand part of the figure. 



sea and deposited in its waters (a). In Fig. 343, the land of the 

 preceding figure is represented as having sunk so as to be partially 

 submerged. A part of the sediments washed down from the remain- 

 ing land are being deposited unconformably on the surface which 

 has suffered erosion. The sediments a are older than the sediments 

 Al, though the latter may be the oldest now accessible. 



Great lapses of time were doubtless involved in the development 

 of the unconformity between the Archean and the Proterozoic 

 formations, but of this interval there is little more than conjectural 

 knowledge. 



