TJie Blue Mound Quartzyte. — Hjtbbard. 163 
THE BLUE MOUND QUARTZYTE.* 
Geo. D. Hubbard, Urbana, 111. 
Over the southwestern portion of Wisconsin and north- 
western portion of Illinois and the adjacent territory in Iowa 
are scattered a number of erosion mounds.f Among these 
is the Blue mound, a somewhat isolated cone with its much 
reduced but similar neighbor called East mound, situated 
about thirty-five miles north of the Wisconsin-Illinois line 
and sixty-five miles east of the Mississippi river. The large 
mound stands in Iowa county and the small one, a mile or 
two east of the former in Dane county. 
Blue mound stands on the water-shed kiiown as "old 
military ridge" lying between the short tributaries of the 
Wisconsin river on its north slope and the long feeders of 
the Platte and Pecatonica on its south slope. In fact it is 
by far the highest part of the divide. 
It is a remnant of former continuous Silurian strata over 
this entire corner of the state. Its top is near the same 
geologic horizon as the summits of the Platte mounds, thirty 
miles southwest; Sinsinnawa mound twenty miles farther in 
the same direction; Charles mound and Pilot knob in Illi- 
nois and Sherrill mound in Iowa. The base is of the same 
formation as the country rock, the Galena limestone. Im- 
mediately and conformably above the Galena occurs the 
Cincinnati shale, and above the latter and capping the hill 
are the beds of the Niagara limestone. 
The Galena limestone is the ore-bearing stratum of this 
region and attains a thickness of two hundred and si.\l\- 
feet at the mound. The Cincinnati shales have been re- 
moved over all the surrounding country but as we approach 
the mound they appear forming a very gradual slopej which 
blends so elosely, the mound slope and the general slope of 
the ridge, it is difficult really to define the limits of the 
mound. 
The characters of the shale are ver\- similar here to those 
of the formation in other out-crops. § No accurate determi- 
*Written from notes taken during the summer of i8gc). 
tWis. Gool. Surv. Chamberlin 1873-79, V. 2, p. 652. 
JWis. Geol. Surv. Chamberlin, 1873-1879 Vol. 2, p. 661. 
§111. Geol. Surv. Northern, V. 5, p. 16. 
Wis. Geol. Surv. Chamberlin, V. 2, p. 685 ff. 
