CHAPTER XII 



THE PROTEROZOIC ERA 



What it represents. The oldest rocks which contain 

 numerous fossils are those of the Paleozoic group. Between 

 these fossil-bearing sedimentary rocks of the Paleozoic and 

 the intricate complex which records Archseozoic time, there is 

 in many places a thick group of systems, partly sedimentary 

 and partly igneous in origin, which represents a vast lapse of 

 time between these two eras. This time is the Proterozoic x 

 era (often called also the Algonkian period). 



PROTEROZOIC ROCKS OF THE LAKE SUPERIOR REGION 



The Proterozoic rocks are nowhere better known than in 

 the vicinity of Lake Superior. In certain parts of this region 



the entire group is divided 

 into four systems which are 

 separated from each other 

 by unconformities. In other 

 localities only three or two 

 divisions are distinguished. 

 Where four systems are 

 known, they are called lower 

 and upper Huronian, Animi- 

 kean, and Keweenawan. 



Basal unconformity. 

 The basal formation may in- 

 clude a conglomerate which 

 contains rounded pebbles of 

 schist and gneiss derived 

 from the Archaean rocks be- 

 neath (Fig. 320). No better 



FIG. 320. Diagram of the contact of 

 Huronian slate with Archaean schist, 

 at a locality in Michigan. (After 

 Van Hise.) 



Which is the younger of the two 

 formations, and what is the evidence ? 



1 From two Greek words meaning "earlier life." 

 322 



