452 THE PENOKEE IKON-BEAEING SERIES. 



At these contacts tlie lowest member of the upper series is the Quartz-slate 

 in different phases. When it is remembered that it is exceedingly difficult 

 and unusual to find actual contacts between unconformable series, even 

 when there is the same certain general evidence of the unconformity that is 

 found in this district, the above number must be considered extraordinary. 

 It is also noticeable that the contacts are widely distributed, and in about 

 an equal nuuiber of cases the schists and the granites are the underlying 

 formations. 



No actual contact between the Southern Complex and the Cherty lime- 

 stone has been found. However, at Penokee gap and in Sec. 23, T. 47 N., 

 R. 45 W., Michigan, the two are exposed close together. At the first place 

 the rock of the Southern Complex is a schist; at the second it is a granite 

 gneiss. At the former the schist and the limestone dip in opposite direc- 

 tions, the schist inclining 30° to the south and the limestone 65° to the 

 north. 



But there is yet further evidence as to the magnitude of this uncon- 

 formity. It is certain that the complex basement ujion which the newer 

 series was laid down was nearly horizontal. When it is remembered that 

 this Southern Comi)lex is here resistant massive granite and there soft 

 foliated schistose rocks, it is plain that the forces of erosion had nearly 

 exhausted themselves, i. e., that a ''base level" of erosion had been nearly 

 attained, before the newer series began to form. When we consider what is 

 involved in this, it strongly reenforces what has gone before as to the great 

 time gap which must have intervened between these older formations and 

 the lower fragmental rocks. The proof of the existence of this nearly 

 level plain lies in the small variation in thickness of the Quartz-slate 

 and the Cherty limestone. The difference of elevation in the basement 

 throughout its whole extent east and west j)robably was not more than a 

 few hundred feet at the time the Cherty limestone began to form. This 

 formation is found here and there in detached exposures for a distance of 

 many miles. Its maxinuun thickness is now 300 feet. In the sea floor 

 there was probably at the tinie when the limestone was deposited no 

 greater variation in elevation than the thickness of this formation as it then 

 existed. 



