394 HISTORICAL GEOLOGY 



divided into two systems (Lower and Middle) by an uncon- 

 formity. 



Resting unconformably upon the Lower Proterozoic (Middle Hu- 

 ronian) is the Middle Proterozoic (Upper Huronian), which resembles 

 the lower system lithologically in that it is composed of similar sedi- 

 mentary rocks and lava flows, but is somewhat less metamorphic. 

 In this system occur the largest and richest deposits of iron in North 

 America. The unconformity which separates the Lower Proterozoic 

 (Lower and Middle Huronian), and Middle Proterozoic (Upper Hu- 

 ronian), is considered by some geologists to be of an importance almost 

 equal to that between the Archaeozoic and Proterozoic systems. 



The closing system of the Proterozoic (Keweenawan), separated 

 from the Middle Proterozoic (Upper Huronian) by an unconformity, 

 differs from the preceding Proterozoic systems in the presence of 

 numerous, and in the aggregate enormously thick lava beds, which ap- 

 parently welled up through fissures (much as in Iceland to-day) and 

 did not flow from distinct volcanoes. The total thickness of these 

 lava flows is estimated at nearly six miles, making this the most no- 

 table time of local volcanism in geological history. In northwestern 

 Minnesota and contiguous portions of Wisconsin there are sixty-five 

 distinct lava flows and five conglomerate beds, none of the former be- 

 ing more than ioo feet thick. In the section cited neither the upper 

 nor the lower limits are known. The maximum thickness of the 

 Keweenawan is estimated at 50,000 feet, of which sedimentary beds 

 constitute about 15,000 feet. Towards the close of the period the 

 igneous outbursts became less frequent, with a corresponding increase 

 in the proportion of sedimentary deposits. The great Lake Superior 

 copper deposits which have up to this time yielded many millions of 

 dollars in profits to their owners occur in the lavas and conglomerates 

 of this system. (The Keweenawan is by some writers considered to 

 be Cambrian.) 



The unconformities which separate the various systems of the 

 Proterozoic in the Lake Superior region are well marked. They are 

 evidenced (1) by basal conglomerates (p. 240) that represent the 

 shores of an encroaching sea, (2) by the irregular erosion surfaces of 

 the underlying rocks, (3) by differences in the amount of volcanism, 

 and (4) by the differences in the metamorphism of the sediments of the 

 overlying and underlying formations. 



In the Grand Canyon of the Colorado the Pre-Cambrian forma- 

 tions (Fig. 368) are more than 10,000 feet thick and differ in many 



