THE PROTEROZOIC ERA 



49 



folded. Sometimes they have been infolded among the Archean 

 rocks. 



Subdivisions of the Algonkian. — In many regions where 

 detailed studies have been made, the Algonkian system may be 

 subdivided into from two to four series separated by distinct 

 unconformities. In some places only one series has been recog- 

 nized. At present no such subdivision into series has a world- 

 wide or even continent- wide application. Generally each of these 

 series shows a thickness of at least a few thousand feet, while the 

 whole Algonkian system has a maximum thickness of many thou- 

 sands of feet, or, according to some estimates, at least ten miles in 



Fig. 23 



Diagram showing the principal subdivisions of the Proterozoic and their re- 

 lation to the Archeozoic in the Lake Superior district. AR, Archean; 

 H, Huronian; A, Animikean; K, Keweenawan. (From Chamberlin and 

 Salisbury's "Geology," permission of Henry Holt and Company.) 



the Lake Superior district. These subdivisions or series of Algon- 

 kian rocks will perhaps be best understood by briefly describing 

 a few of the better known regions. 



Lake Superior District. — One of the best and most carefully 

 studied Algonkian districts in the world is that around Lake 

 Superior. Algonkian rocks are there divided into four distinct, 

 largely sedimentary series separated from each other by uncon- 

 formities, and named Lower Huronian, Middle Huronian, Upper 

 Huronian (Animikian), and Keweenawan (Fig. 23). At some 

 localities not all of these series are represented. The relations of 

 these series to each other and to the Archeozoic below and Pale- 

 ozoic above are brought out in the tabular arrangement given on 

 page 42. As indicated by the unconformities, the deposition of 

 each series was succeeded by emergence of the region accompanied 

 by erosion, and this in turn followed by submergence accompanied 



