EXTENT AND AGE OF CAP-AU-GRES FAULT 63 
When working in Missouri I had also taken occasion to trace 
the Cap-au-Gres fault, the features of which are so well dis- 
played on the Mississippi river above the mouth of the Illinois 
river, from near Folley Station through Lincoln and Pike 
counties. Now all these apparent anomalies of which mention 
has been made lie in a slightly curved line extending from the 
mouth of the Illinois river to Leon, in Decatur county, Iowa, 
Indeed they prove to be expressions of some line of notable 
displacement rather than of a line of unusual flexing. 
Eastward from the mouth of the Illinois river the fault-line 
passes through the city of Alton, the prominent bluff overlook- 
ing the town being in fact a fault-scarp. Although not vet 
actually traced on the ground beyond Alton the distribution 
of the coal mines and other features indicate in no unmistak- 
able way that it extends far beyond. The line seems to pass 
about three miles north of Edwardsville, about twelve miles 
south of Vandalia, near Louisville, through Olney and Law- 
reneeville, at the southernmost end of the Illinois oil fields, to 
Vincennes, Indiana. Between Vandalia and the Indiana boun- 
dary the line is parallel to the anticlinal axis of the Oshawanpe 
Hills, a marked structural range crossing southern Illinois and 
forming the eastward prolongation of the Ozark uplift of south- 
ern Missouri and northern Arkansas. 
Prom Leon, Iowa, to Vincennes, Indiana, the distance is 400 
miles. At the Mississippi river the difference in level of the 
same layers on the two sides of the fracture is more than 1,000 
feet. The Cap-au-Gres fault is the most remarkable line of 
displacement in the entire Mississippi valley. At the Sand- 
stone Headland the movement is probable near maximum. To- 
wards either end the amount of displacement becomes grad- 
ually less and less until finally in north Missouri and southern 
Iowa there is no fracturing of the rocks at all, the vertical 
movement finding expression in a sharp monoclinal fold. 
The typical structural features of this great line of displace- 
ment as exposed at the Cap-au-Gres on the Mississippi river, 1 
have already, described. 1 Near the fault the rocks are upturned 
so that in a distance of about one mile along the river almost 
the entire Paleozoic section from the Cambric to the Carbonic 
1 Proc. Iowa Acad. Sci., Vol. V, p. 58, 1898. 
