GENERAL GEOLOGY OF THE DISTUICT. 453 



At the beginning of the deposition of the Quartz-slate member of the 

 Penokee series proper the case was even more striking. Tliis luvei-, aver- 

 aging 300 or 400 feet in thickness, but in one phice reaching twice this 

 amount, at no point is known to be nuicli less than 300 feet in thickness. 

 It forms a continuous belt throughout most of the extent of the series, and 

 where missing has been swept away b}^ erosion, as have the higher mem- 

 bers at such places. It is certain, then, that the variation in elevation of 

 this ancient land surface at this time was little or no more than 300 feet, 

 except, perhaps, in the eastern pai't <>f tlie Eastern area, where it is not clear 

 that the slates there occurring as the lower la\ers of the Penokee series 

 are the equivalents of the Quartz-slate to the westward. 



A rock basin composed of diversified crystalline sc-hists and massive 

 rocks 90 miles in diameter and having no elevations yreath' exceeding- 300 

 feet is rare, and it is universally believed that the complex areas in which 

 this is the case have been subjected to enormous denudation. The amount 

 of erosion in such regions is often compared to that necessary to reduce 

 lofty mountains to mere stumps. Before the l)eginning of the deposition of 

 the Penokee series proper it is clear that the Southern Complex was tlms 

 brought nearly to a plane. 



To summarize, the proofs of unconformity above the Southern Com- 

 plex are as follows: First, belts of sedimentary rocks strike across the 

 country, being now in contact with one variety of underlving rock, now in 

 contact with another, always keeping their course, never being i)enetrated 

 or interfered with by any of the southward lying rocks (excluding, of 

 course, the later basic dikes), wliether schistose or g-ranitic intrusions in the 

 schists. Second, these underlying- rocks are either massive ones whii-li 

 are presumably igneous or are schists in which the extreme of foliation and 

 crystalline character is found, while the overlying Penokee rocks are 

 plainly water deposited sediments. Third, in ten places above the Base- 

 ment Complex are basal conglomerates or recomposed rocks. Seven of 

 these places show the actual unconformable contacts. The detritus is 

 mainly identical in character in each case with the material of the rock 

 upon which it rests, showing that the basement rocks must have reached 

 their present condition before tlie formation of the lowest overlying 



