GEOLOGY OF IsORTHER^ST WISCONSIN. 43 



grained white quartz with a vitreous coating and slaty siliceous 

 schist occur, showing a strike nearly east and west, and dip of sixty- 

 six degrees to the north. The quartz represents the lowest mem- 

 ber of the Penokie system examined by the party in 1873. Upon 

 examining the opposite wall of the gorge siliceous marble was dis- 

 covered for the first time to be one of the beds of the Penokie sj^s- 

 tem, lying below the iron bearing beds."*" A similar arrangement 

 has long been known to exist in the Huronian of the Marquette 

 district, which has led to the susjncion of its existence in Wiscon- 

 sin. The thickness of the siliceous marble is about fifty feet. It 

 is usuall)^ fine grained and grayish in color. Small crystals ofcal- 

 cite and dolomite however can be observed irregularly disseminated. 

 An anal3^sis of a specimen taken from the ledge afforded me the fol- 

 lowing result: 



Percent. 



Carbonate ot Lime , 50 . 52 



Carbonate of Magnesia 33 . 41 



Insoluble Matter 13 .85 



Oxide of Iron 1.70 



Undetermined .52 



Total 100.00 



The analysis shows that the proper name for the rock is siliceous 

 dolomitic marble. In the Marquette region the Morgan furnace 

 limestone but very little purer than this has been extensively used 

 as a flux. One hundred feet southeast from the exposure of sil- 

 iceous marble, there is a large ledge of gneissoid granite showing a 

 well defined dip ot seventy-seven degrees to the south, and strike 

 of north, seventy-five degrees west. In following the strike Avest, 

 one passes within twenty-five feet of the outcrop of siliceous mar- 

 ble which has a northerly dip. Between one and two hundred 

 feet south, on the line of the railroad, other large exposures of 

 gneissoid granite are found having essentially the same bedding as 

 that mentioned above. When the railroad cut is completed at this 

 locality, the absolute junction of the Laureutian and oyerlying 

 Huronian will doubtle^^s be exposed. There can be no doubt of the 

 unconformability of these formations, approaching each other as 



* I will say in this connection that the facility for making observations at tliis lo- 

 cality have been greatly increa,sed since Professor Irving's examination of the " Gap." 

 Excavations have been made at the gorge for a railroad bridge and the earth and 

 roots which formerly overhung the face of the wall removed. The rocks are now 

 plainly exposed and are easily accessible. 



