268 HOBBS— THE EVOLUTION AND THE ' [April 24, 



Writing in 1902 Professor John Milne, who has done so much 

 to advance seismology, gave expression to his views upon the cause 

 of the larger and smaller earthquakes :^^ 



The earthquakes to be considered may be divided into two groups — first, 

 those which disturb continental areas and frequently disturb the world as a 

 whole, and secondly, local earthquakes which usually only disturb an area 

 of a few miles radius and seldom extend over an area with a radius of 100 

 or 200 miles. 



These former I shall endeavor to show are the result of sudden accelera- 

 tions in the process of rock-folding accompanied by faulting and molar dis- 

 placements of considerable magnitude, whilst the latter are for the most 

 part settlements and adjustments along the lines of primary fractures. The 

 relationship between these two groups of earthquakes is therefore that of 

 parents and children. 



Professor Milne's studies of " distant " earthquakes had revealed 

 the fact that the world-shaking earthquakes most frequently occur 

 upon the floor of the ocean. 



When a world-shaking earthquake takes place, and its origin is sub- 

 oceanic, we occasionally get evidence that this has been accompanied by the 

 bodily displacement of very large masses of material. For example, sea- 

 waves may be created which will cause an ocean like the Pacific to pulsate 

 for many hours. 



To indicate the grand scale of the mass movements of the crust 

 upon the continental areas, a list of twenty-two larger disturbances 

 was compiled by Milne and the following important conclusions 

 drawn : 



If it can be admitted that world-shaking earthquakes involve molar dis- 

 placements equal in magnitude to those referred to in tlie preceding list, . . . 

 then, in the map showing the origins of these macroseismic effects, we see 

 the districts where hypogenic activities are producing geomorphological 

 changes by leaps and bounds. 



The sites of these changes are for the most part suboceanic troughs. 

 When they occur, the rule appears to be that a sea becomes deeper, whilst 

 a coast-line relatively to sea level may be raised or lowered. For nearly all 

 the regions of the world where they take place we have geological and not 

 unfrequently historical evidence that the more recent bradyseismic move- 

 ments have been those of elevation. This elevation, however, only refers 

 to the rising of land above sea-level, while the mass displacements seem to 

 be accompanied by sudden subsidences in troughs parallel to the ridges where 

 rising has been observed. In short, at the time of a large earthquake, two 



" " Seismological Observations and Earth Physics," Geogr. Jour., Vol. 

 21, 1903, pp. 2, 9, II. 



