THE FARTHQUAKE OF MAY 26, 1909 155 
MINNESOTA 
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ISOSBISMALS OF THE HARTHQUAKES IN THE Upper Mississippi Valley, May 26, 1910. 
more regularly undulating nature of the free and gradually vanishing 
oscillations of the earth. 
The greater number of earthquakes are now known to be due to 
slipping of enormous blocks of the earth, along fissures or joints of 
great depths, and thus forming the dislocations known to geologists as 
faults. In the case of the earthquakes with two maxima of disturbance, 
the slipping occurs first at one point in such a fissure, and then at 
another. There can be no doubt that this Illinois earthquake was of 
the nature of such a compound slip, although the exact position of the 
fault can not be correctly located from the data at hand. In most 
descriptions of the shock no mention is made as to whether there was 
one maximum or more. Such particulars were naturally overlooked. 
The people of the upper Mississippi Valley are not trained in making 
observations on earthquakes. Nevertheless, nine observers make men- 
tion of more than one commotion. One account from each of eight 
localities states that two distinct shocks were felt. These places are 
Bushnell, Canton, Champaign, Chicago, Geneva and Sterling in IIli- 
nois, and Davenport and Dubuque in Iowa. In the latter place the 
first disturbance lasted about ten seconds, after which there was a short 
pause and then again a shock of short duration. But the reports from 
Chicago, Springfield and Champaign, which places lie on the other side 
of the mesoseismal area, all agree in stating that the first shock was of 
brief duration, and that the second lasted several seconds. One ob- 
server is reported as having noted three distinct shocks, and this was 
Professor W. H. Hobbs, at the time on a visit in Madison, Wis. He is 
