138 THE PENOKEE IRON-BEARING SERIES. 



17. Flinty chert. Specimens 941.'^ (slide :H)r,-l), 9414 (slide .3055), 9415 (slide 

 3056), all from 2."")() N., l.'^.W W., Sec. 17, T. 47 N., R. 44 W., Michigan. 



These si)eciniens closely resemble 94.30 and 9434 in 10. Immediately overljing 

 them comes the tVagmental slate that forms the first member of the overlying series. 



The sections of these specimens show in part numerous quartz fragments buried 

 in a matrix of minutely crystalline silica, each quartz fragment having about it a halo 

 of added material. Other jiortions are without any fragmental quartz, being made 

 up of minutely crystalline silica ill all respects like that of 2056 and 3131 in 10. In 

 still other jilaces, however, are areas of very coarse grained quartz, the individuals of 

 which interlock intricately and present an appearance of true vein quartz. These 

 areas and bands seem hardly to be vein quartz in the sense that they have been inti-o- 

 duced wholly subsequently to the remainder of the silica. 



Prominent exposures. — The various exposures upou which tlie existence 

 of this member as a distinctive liorizon is based are h>cated carefully on 

 the accompanying detailed maps (Pis. v to xiii) so that it will not be desir- 

 able here to give a description of each of them. A few words, however, 

 with i-egard to the greater exposures, and ])articularly with regard to those 

 which illustrate best the general nature of this member, may be of ad- 

 vantage. 



Beginning at the west, we may note first the several exposures of lime- 

 stone running along flie south line of Sees. 15 and 16, T. 44 N., R. 5 W., 

 Wisconsin, in the vicinity of Marengo river. These exposures show relatively 

 little of the cherty material, being mainly made up of an unusually coarse 

 crystalline white or greenish white limestone, which is often exceedingly 

 tremolitic; bands occurring in the rock which are composed entirel}' of 

 interlocking blades of tremolite. The large exposure indicated on Plate v 

 as occurring not far north of the middle of Sec. 24, T. 44 N., R. 4 W., 

 Wisconsin, shows a coarsely crystalline limestone very similar to that just 

 described, containing but little chert, but seams and launches of greenish 

 white tremolite are particularly prominent. The extent of this exposure 

 appears to indicate that we have here a considerable thickness for the lime- 

 stone member. In the vicinity of Penokee gap the exposures of this mem- 

 ber are those immediately under the railroad bridge across Bad river on the 

 south side of the gap, in the SE. i of the NW. \, Sec. 14, T. 44 N., R. 3 W., 

 Wisconsin.' Bad ri\er here runs with an easterly course through a narrow 



' Geol. of Wis., v.d. in, 18WI, p. 1013. Alsn Atlas to the Ueol. nf Wis., PI. xxiil. 



