I 

 264 HOBBS— THE EVOLUTION AND THE [April 24, 



and the wonderfully improved maps which have been brought out 

 in recent years by the United States Geological Survey and by Euro- 

 pean surveys have been secured through the elimination of the 

 process of averaging and " rounding off " of angles. Significant 

 character is thus taking the place of a lack of expression in the 

 older maps. 



In a similar way the isoseismals and coseismals, which have 

 assumed to represent the distribution in space and in time of the 

 seismic activity of a district, have through averaging of results 

 removed all true expression of seismic distribution. It is likely, 

 however, that this method will yet, at least for a number of years, 

 effectually retard the natural progress of seismology. 



The Evolution of the Fault Block Theory of Earthquakes. — It 

 would be incorrect to state that no progress was made in seismic 

 geology during the last half of the nineteenth century, but it would 

 be only the truth to say that such progress as there was, was 

 achieved in spite of and almost in defiance of the orthodox doctrine 

 of seismology. Nine out of ten reports upon special earthquakes 

 made during that period have included only the maps of isoseismal 

 and coseismal lines, to which has been added a computation of the 

 depth of the supposed origin. 



It is now proposed to trace the development of the tectonic con- 

 ception of earthquakes as it has grown into the fault-block theory 

 of the present day. To the Austrian school of geologists and to its 

 leader, Eduard Suess, must be credited the pioneer work upon the 

 geology of earthquakes. The discovery of the localization of heavy 

 shocks along definite lines, or the recurrence of epicenters (surface 

 loci of heavy shocks) along such lines, has been a characteristic of 

 the Austrian method, which dates from a paper published by Suess 

 in 1872. Such lines in the surface, generally approximating either 

 to a right or to a broken line, were in some cases identified with the 

 traces of fault planes and in others were shown with much proba- 

 bility to be the course of such displacements. Here, then, was the 

 first important recognition of the tectonic nature of earthquakes, 

 and, as a consequence, the Austrian school of seismologists has since 

 endeavored to examine earthquakes in the light of the geological 

 structure of the affected region. 



