GEOLOGIC.KL PAPERS II3 



lines suggests that there was one distinct epicentre from which 

 the waves spread in the form of ever-widening circles. The 

 earthquake was most severe within the area enclosed by the 

 isoseismal whose intensity- is six. The cities in this area which 

 report the most violent effects are Morris and Aurora, for which 

 the intensities are more nearly seven than six. 



While the principal epicentre is the one just described, there 

 appears to be also a secondary- epicentre whose intensit\- is five. 

 This is at Dixon. Illinois, about fift\-five miles west of the first 

 epicentre. With the data on hand it is difficult to determine 

 whether this is a distinct epicentre of less intensity- or whether 

 Dixon should be included in the area enclosed by the isoseismal 

 whose intensity is five. The dotted line indicates the direction 

 which this isoseismal would take in the latter case. That there 

 were two epicentres appears to be supported by obser\ations on 

 the number of shocks reported to have been felt at different locah- 

 ties. The majorit>- of these obsen^ations state that there were 

 two distinct shocks — one of greater intensity, preceded or fol- 

 lowed by one of less severity". 



Obser\'ations upon the directions of motion of the earthquake 

 waves are reported from four different localities — Chicago, Mil- 

 waukee. Elgin and Rock Island. The directions are indicated on 

 the map by means of arrows drawn through the respective cities. 

 While the arrows do not intersect at a common point, yet in a 

 general way they converge toward the region of greatest dis- 

 turbance. 



The area sensibly affected by this eanhquake covers about 

 40,000 square miles. It is possible that the earthquake did not 

 extend for any considerable distance beyond the limits of the 

 sensible area. In a letter. Professor J. B. Goesse. S. J., states 

 that the seismograph belonging to the meteorological observaton.- 

 of the St. Louis Universit}.' in St. Louis did not record any x-ibra- 

 tions which could with certaint}- be referred to this earthquake 

 disturbance. Francis J. Glover. S. T.. of Brookh-n College, Brook- 

 lyn. Xew York, states that if their seismograph recorded the 

 earthquake the record was obliterated by local disturbances 

 caused by nearby railroad traffic. 



The observations on the time of occurrence and duration of 

 the eanhquake do not warrant any deductions. The time of 

 occurrence is variously stated from 10:15 to 10:35 ^- ^•' while 

 the reports on duration range from one second to three minutes. 



It is interesting to compare this earthquake with the one which 



