130 THE PENOKEE IHONBEAliING SERIE8. 



at the base of the sUite. It is shown below that whatever may be 

 the orifi'iu of the chert of the hinestone beU, it had phiiiily in tht; main 

 reached its present i-ondition b<ifore the beginning' of tliosc ])rocesses oi 

 mechanical sedimentation by wliicli the Quartz-slate memljer was accumu- 

 lated, since fragments of tlie chert are connnonly included within the slate. 

 It is more than probable that the forces of erosion which accunmlated the 

 fragmental slate may have in places completely swept away the underlying 

 limestone and chert. 



Thickness. — The thicknesses of this member, noted at different points, 

 vary between somewhat wide limits. At Penokee gap the entire surface 

 width of the chert and limestone together is 125 feet, which coiTesponds to 

 a thickness of about 113 feet. On the other hand, in the SE. J of Sec. IS, 

 T. 47 N., R. 44 W., Michigan, there is a continuous surface exposure of 

 limestone and chert of such widtli that the thickness of the member in this 

 place must be at least 300 feet. This is the greatest thickness that has any- 

 where been oliserved, but there are indications that in some places the 

 member ma}- at times l)e mucli thimier than is indicated by the Penokee 

 gap section, as would l)e expected, since it is at times wanting altogether. 



Petro()raphical cluirddcr of the limestone. — Externally the limestones of 

 this lielt vary very considerably in appearances, the variation being due 

 mainly to a varying coarseness of grain, which is at times so small that tlie 

 i-ock is not far different in appearance from any compact earthy limestone 

 of the fossiliferous series. On the other hand, it at times presents a very 

 distinct and somewhat coarsely crystalline aspect, though the more com- 

 pact kinds predominate. 



Tlie color generally ranges from white to gray, iron-stained portions 

 being rather unusual. Thin blades of white tremolite may occasionally be 

 seen on tlie sm'f;ice of a specimen. Analyses of tlie solul)le portions of 

 these limestoues indicate that they are always strongly magnesian ; indeed, 

 as shown by the following analyses by Mr. W. F. Hillebrand, of tlie U. S. 

 Geological Survey, they have so much magnesia as to justify the applica- 

 tion to them of the name dolomite. No. 1 is from near Siniday lake, 

 (specimen 9405) SE. \ Sec. IS, T. 47 N., R. 44 W., Michigan; No. 2 

 (specimen 9677) is from the NW. \ Sec. 22, T. 44 N., R. 5 W., Wisconsin. 



