THE IRON-BE AEING MEMBER. 189 



Quartz-slate ineinber climbs higher and higher oii the southern face of the 

 ridge, all of which it makes up by the time mount Whittlesey is reached. 

 Correspondingly, the Iron-bearing member creeps farther and farther down 

 the northern slo])e. At Tylers fork this change has gone on so far that 

 the crest of the ridge is now entirely within the quartz-slate, while east of 

 that stream and all the way to the West l)rancli of Black river the crest 

 of the ridge is in the granitic rocks belonging to the Southern Complex. 

 In this area the Iron-bearing member continues to creep down the 

 northern face of the ridge until it has the greater ])ortion of its width in the 

 low ground to the northward. East of the West Itranch of Black i-iver 

 for a short distance the ridge again lies within the iron belt, which here 

 contains an unusually large amount of resistant jasperv material. The 

 changing relation thus indicated as obtaining between the j)ositions of the 

 ridge and that of the outcrop belt of the Iron member lias Ijeen explained 

 on a previous page as a result of a variation in tlie niineralogical character 

 of the member, which, where resistant, forms the u})pcr portions of the 

 ridges, while in other places the rocks to the south being more resistant 

 the course of the iron ])elt lies in the lower ground to the nortli. 



ThickHeas.—lt has already been said that the outcrop belt of the Iron- 

 bearing mend^er has a singularly constant width from 800 to l,00tl feet for 

 all of the distance between the western extremity of the Penokee ranire to 

 the middle of T. 47 N., K. 4(; W., Michigan. Tlie thickness must be yet 

 more constant than is indicated by these figures, the variations between the 

 limits indicated being generally exjdicable by a somewhat changing degree 

 of northward inclination. Throughout this distance the actual thickness of 

 the formation can not vary much on either side of 850 feet. How tar the 

 great increase in surface width, which has already been indicated as 

 ol)taining in the eastern portion of T. 47 N., K. 4() W., Michigan, and 

 farther to the east, is due to an increase in actual thickness is exceedingly 

 difHcult to tell. Some of the increased width is plainly the result of an 

 unusually low angle of northward inclination. More of it is evidently due 

 to the intercalation of erujjtive greenstone sheets; Init after tliese two 

 causes of widening have been considei'efl, liow mncli remains to be 

 accounted for by an increased thickness of the Iron-bearing member we 



