172 



GEOLOGY. 



in section, the general relations of the Proterozoic systems to the 

 Archean (JR) below, and to younger formations (€) above. The 



-^ 





lA 





Fig. 51. — Diagram showing a common surface relationship between Archean (JR), 

 Proterozoic (Al), and Cambrian (€). The Proterozoic (Algonkian) formations 

 appear at the surface between younger and older formations. 



same relations are shown in ground- plan in Fig. 52. While the relations 

 shown in these diagrams are common, there are areas of Archean not 



mi. 



Fig. 52. — Diagrammatic map, showing in ground plan the relations expressed in 



section in Fig. 51. 



surrounded or bordered by exposed Proterozoic formations, and areas 

 of the latter not associated with exposed Archean. The former rela- 



Fig. 53. — Diagram showing how Proterozoic rock (Al) may fail to outcrop about 



Archean (JR). 



tionship is explained by Fig. 53, where a later formation is represented 

 as lapping completely over the Proterozoic (Al), Younger formations 



