Page:Popular Science Monthly Volume 77.djvu/161

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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 mention 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 Illinois, 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 observer 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