128 THE PEXOKEE IRON-BEARING SERIES. 



terials constitute a well marked belt, which is sharply separated, not only 

 from the granites and schists to the south of it, liut also from the fragmen- 

 tal slates immediately above, from which it differs in l)eing almost free 

 from an intermixture of fragmental material. 



Geofiyaphiral (listrihidion. — It is difficult to define very accurately the 

 geographical distribution of the limestone member. Certainly in a number 

 of places it is cntirelv wanting, since the strongly marked .slate belonging 

 immediatelv above it is found directly in contact with the granites and 

 schists which belong below it. When the atlas sheets of the Geology of 

 Wisconsin were drawn, this belt was represented as having a greater 

 degree of continuity than is indicated on the maps that accompany the 

 present volume, exposures distant from one another having been relied on 

 to indicate a continuity, so that, .substantially, this member was repre- 

 sented as continuous except where it was certainl}- known tit be wanting. 

 Our later experience having taught us tluit this member may thin out and 

 disap[)ear quite suddeuK', and that it is lacking more often than was for- 

 nierlv supposuil, we have })ursued just the opposite course in mapping it; 

 that is to say, we have maj)ped it as occuring only in those places where 

 exposures demonstrate its existence. 



The detailed maps of Pis. v, vi, viii, x, xii, and xiii of, the present 

 vcdume, which show the exact positions of all of the exposures belonging 

 to the various belts iif the Penokee series, will serve to indicate to the 

 reader the exact facts which we have hail at command in deciding as to 

 the degree of continuity possessed by this member. This member then 

 is indicated on om- maps as appearing in six detached portions in the 

 distance between Atkins lake, T. 44 N., R. 5 W., Wiscon.sin; and the Little 

 Presfpie Isle river, T. 47 N., R. 44 W., Michigan, beginning with the west- 

 ernmost of these areas, it is to be said that, so far as our knowledge goes, 

 it may extend some miles farther Avest than the maps indicate, since we are 

 without exposures to prove either its continuity or its absence. The eastern 

 end of this westernmost area, however, is sliarpl}' defined liy actual expos- 

 ures, as indicated on PI. v, the granite and siliceous slate member of the 

 iron series coming directly into 'contact with one another along the course 

 of the Marengo river, in the SW. \ of Sec. 15, andNW. \ Sec. 23, T. 44 N., 



