DISPLACE.AIENT OF THE POLES 891 



AlR-OUAKES. 



What is true of the sea is in an even greater measure true of 

 the atmosphere. This, Hke the water, is too mobile to permit the 

 setting up of strains within its mass, and so bary seismic disturb- 

 ances cannot originate in the atmosphere any more than they can 

 in the hydrosphere. Such disturbances may, however, be com- 

 municated from the baryseismic disturbances of the land. Origin- 

 ally induced pyroseismic disturbances are, however, as readily pro- 

 duced in the air as in the water, and they are more readily trans- 

 mitted, owing to the greater mobility of the atmosphere over that 

 of the water. 



Periodicity of Earthquakes. 



Periods of strong seismic disturbances (macroseisms) are 

 known to alternate with periods of relative quiescence, or only 

 minor disturbances (microseisms ). In Japan the seismic periods 

 recur about once in thirteen years, though observations at Kioto in- 

 dicate a period as short as six and one-fourth years. Too little 

 is yet known to warrant predictions of recurrences of earthquakes. 



Movements Due to Displacement or Migration of the Poles. 



The theory of pole migration, or the shifting of the earth's axis 

 of rotation with reference to the earth itself, together with the 

 accompanying changes in the form of the earth, in distribution 

 of land and sea and in climatic belts, has ever proved attractive 

 to speculative geologists who sought for means of accounting for 

 the ascertained variations in the surface characters of the earth in 

 the past. Thus, evidence is accumulating which points to a widely 

 different position of the earth's axis during Palaeozoic time from 

 that which it held during the Alesozoic and subsequent periods. 

 This evidence is furnished in part by the occurrence of mild cli- 

 mate, and even of tropical vegetation, in regions having an arctic 

 climate at present, and in part by the presence of glacial deposits 

 where tropical conditions prevail to-day. It is further found in 

 the occurrence of sediments which indicate the existence during 

 Palaeozoic time of easterly winds where now the westerlies prevail. 

 (See ante, Chapters II and XIV. also Fig. 128, page 636.) 



A wealth of biological evidence has been accumulated which 



