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IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
terranes is fully represented and is in sight at one time. I re- 
call no other instance of the kind on the whole continent. 
The magnitude of the displacement is best shown in diagram 
(Plate III) in which the distance between the two parts of the 
Coal Measures is indicated to be approximately 1,123 feet. This 
figure does not represent the total movement. It is only the 
actual vertical movement. The lateral movement is doubtless 
much more ; how much it is not possible at the present time to 
state. In the case of the crustal rupture producing the San 
Francisco earthquake a decade ago the maximum vertical trans- 
location was about four feet, while the horizontal component was 
more than twice this amount. Perhaps about this ratio obtains 
in the Cap-au-Gres instance. 
One of the remarkable features of this fault is the tremen- 
dous extent of the “drag.” In the west bluff of the Mississippi 
river this is shown to be not less than 300 feet. Many faults of 
greater throw do not display any appreciable drag. 
This recognition of the wide extent of the Cap-au-Gres dis- 
placement has a profound effect upon the general mapping of 
the states through which the line passes. A broad belt reach- 
ing east and west entirely across the state of Illinois now sorely 
needs notable rectification of all of the formational boundaries. 
A similar wide belt traversing the state of Missouri also re- 
quires complete readjustment of the control on the areal dis- 
tribution of the rock terranes. In Iowa., where far less map- 
ping was done in the office and where the field work was more 
painstaking, the published maps demand no material revision. 
It is singular that in the other states so conspicuous a feature 
should be so long so completely overlooked, especially since the 
cue is so plainly given. It is instructive in this connection to 
peruse the field notes of J. A. Udden 2 on the tracing of the 
Shoal Limestone in western Illinois. After following the out- 
crops of the formation southward entirely across Macoupin 
county he suddenly loses all signs of them; and they do not 
reappear to him until he reaches the south edge of Madison 
county, fifteen or twenty miles away. Now this interval where 
he is unable to detect the Shoal rock is right in the great fault 
belt and the limestone no doubt lies several hundred feet be- 
neath the surface of the ground. If at the last point of ex- 
2 Illinois Geol. Survey, Bull. 8, p. 120, 1907. 
