17<) The American Geologist. September, 1894 
posed <>i' sand and clay, with no calcareous material. Now. 
the St. Peter sandstone in this region is always distinctly 
stratified and free from day. This particular stratum is more 
like the geest or residual material which has collected over 
the sandstone in the Driftless Area. Moreover, it contains 
reddish brown oxide of iron in patches, which iron stain re- 
sembles that found in surface clays at the present day. and 
differs decidedly from the iron stains so common in the sand- 
stone. Over this loose sand there area few inches of dark 
brown stratified clay, which would be exactly imitated were our 
present black soil washed away and redeposited b} r currents 
of water. Over this lies the fossiliferous Trenton limestone. 
•Nowhere in this region have I found such strong evidences of 
a land surface and soil in Silurian times. The Trenton lime- 
stone is here about 40 feet thick. The Buff limestone is 
present as indicated b} T outcrops and well sections, in all di- 
rections from this locality. As there is no stratigraphic break 
between the sandstone and Buff limestone when it is present, 
the land surface here shown to have existed must have been 
small, probably an island a few square miles in area. Similar 
spots of absence of the Buff limestone have been reported 
from the Rock river area and other places in northwestern 
Illinois, and there thus seems to have been a slight but ex- 
tended disturbance in this region during the earlier portion of 
the Trenton period. 
The Galena li u<<>!i>,ic This underlies all the higher upland 
country, but it is scarcely anywhere exposed. Its thickness 
here is not over 100 feet, while in Stephenson county to the 
north it is 160 feet, and about Galena and Dubuque 350 feet. 
Deformations of the Area. 
The area whose geology is under discussion is crossed by 
three anticlinal axes. Tin- main axis trends from northwesl 
to southeast and is a continuation of the Grand de Tour — La 
Salle anticlinal, which is the chief axis of northern Illinois. 
This anticline in the Elk Horn district is not very prominent, 
the strata dipping in both directions from the crest at a rate 
not exceeding 30 feet per mile. This alone could not have 
brought the St. Peter sandstone to light: but it is the inter- 
section of this with two east and west anticlinals which 
has so elevated the formation that stream erosion has laid it 
