1909.] OUTLOOK OF SEISMIC GEOLOGY. 291 



far been possible only within this circum-Pacific zone, which, at the 

 time, is passing through a remarkable seismic history. It is little 

 likely that any such sudden relief of strain will take place again in 

 the same zone before a considerable period has elapsed. 



Yet, outside this zone and within our own country, earthquakes 

 of the first order of magnitude have visited the lower Ad^ississippi 

 Valley, the coastal plain in South Carolina and the valley of the 

 St. Lawrence during the brief period that the country has been 

 occupied by whites. Of these sections of country, as of most 

 others, the only safe prediction that can now be made, is that dis- 

 tricts already visited by historical destructive shocks, as well as 

 some others, notably New England and the Middle States, will 

 eventually sufifer from disastrous earthquakes. To the time of such 

 visitations we have not even a clue. 



Periodicity of Earthquake Cycles. — The " letting down "of the 

 potential energy of the system of stresses within the circum-Pacific 

 belt, as brought out by the events of 1906-7, is, in the writer's 

 belief, as regards its close sequence, an event without parallel in the 

 history of seismic geology. Something approaching it appears, how- 

 ever, to have been in operation within a somewhat longer period in 

 the other great seismic belt of the globe. Making all due allowance 

 for the fact that our quite recent study of distant earthquakes has 

 greatly extended our horizon, it still seems necessary to conclude 

 that the present is a time of very exceptional seismic intensity. 



So soon as we admit the planetary scale of these seismic dis- 

 turbances and explain them as a result of mountain growth upon 

 the borders of the continent, we are led to expect the existence of 

 such maxima and minima of seismic intensity. If now we examine 

 the history of earthquakes in those countries possessing the longest 

 records, we find evidence in support of this view. The stronger 

 earthquakes in Japan, which are on record for a period of fifteen 

 hundred years, betray a strong tendency to group themselves. The 

 154 heavy earthquakes recorded in that country since the beginning 

 of the fourteenth century may be divided more or less definitely into 

 41 groups separated by average intervals of 13I years. In Kyoto 

 a complete record has been kept for a thousand years. Here there 

 was a strong maximum of destructive and strong earthquakes be- 



