THE PROTEROZOIC ERA. 165 



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

 able relations of the Archean and the Proterozoic, where seen, is illus- 

 trated by Figs. 49 and 50. Fig. 49 represents an area of land composed 

 of Archean rock in such a position as to suffer erosion. The sedi- 

 ments derived from it are washed down to the sea and deposited in its 

 waters (a). In Fig. 50, 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 remaining land are being deposited on the sur- 

 face which formerly suffered erosion. These sediments (Al, Fig. 50) 

 overlie those deposited at the earlier stage. The latter (a) are the 

 older, though the former may be the oldest now accessible. 



Subdivisions. 



No classification of the formations of the Proterozoic era has been 

 widely accepted as of general application, but where these rocks 

 are best known, at least three great unconformable series or systems 

 are referred to this era. These several divisions are of such thickness 

 and extent that their local names may well be taken for the great 

 subdivisions of the rocks of this era. In the order of age, these systems 

 are (1) the Huronian, (2) the Animikean or Penokean, and (3) the 

 Keweenawan (p. 160). The first system is named for the region north 

 of Lake Huron, where the rocks were first differentiated. The names 

 of the second system come from localities in northeastern Minnesota 

 and northwestern Wisconsin respectively, where the rocks are well 

 developed, while the name of the third system is derived from the 

 Keweenaw peninsula of northern Michigan, on the south shore of 

 Lake Superior. 



Each of the systems in this region measures thousands of feet in 

 thickness, but it is important to note that, thick as they are, they 

 do not constitute a complete record of the Proterozoic era. The 

 unconformities between them show that, after the formation of each, 

 there was a disturbance of relations between the sources of the sedi- 

 ments (the lands) and the sites of their deposition (chiefly the sea). 

 Each unconformity appears to mark a prolonged period during which 

 erosion was in progress in the region where the formations are exposed, 

 and deposition somewhere else. The sediments deposited at this 

 time are probably buried under later formations, either within ttie 

 continent or about its borders, and remain inaccessible. The intervals 



