53<3 



KNOWLEDGE & SCIENTIFIC NEWS. 



[September, 1906. 



E^arthquake Areas. 



As students of seismology are well aware, the earth's 

 surface oonlains many lar^e areas where, owing to the 

 distriljulion of pressure of the strata and the compara- 

 tive instabihty of ("quihbrium among^ the strata, there is 

 a continual liahilitv to those movements of the earth 

 which are called earthcjuakcs. These areas have been 

 mapped out by Professor John Milne and other ob- 

 servers, and a chart is annually presented to the British 

 Association showing the number of earthquakes that 

 have originated in these areas, and the records of which 

 have been sent to Professor John Milne's observator\ at 



Association, ySth lief-ort, York, 190'). 



crest of the range to what we may figuratively call its 

 foundations is much greater, and in speculating on the 

 causes of earthquakes we may regard the upper strata 

 on this great pile of stratified rocks as tending to slip 

 over one another. W'e may further regard the tend- 

 ency as accentuated by the fact that the lowermost 

 strata, being under very great pressure, are approaching 

 that condition when they tend to lose their solidity and 

 become viscous. Laboratory experiments have shown 

 that given sufficiently high pressure iron can be made 

 to flow. We may presume that the lower rocks will 

 flow also. Therefore, a great mountain range situated 

 next to an ocean is not in stable equilibrium, and a 

 movement sometimes of earth-shaking magnitude may 

 be precipitated by very many apparently slight causes. 



Biilish Association, ■jbiU lief'ort. York, 190'). Tlie Large EirtUqujkes of i< 



Origins for 1905 ar<? indicated by their B A. Shide Kcgister nninber. Earthquake districts .ire indicated 



number of earthquaties which since iSgg have originated from these is expressed in large miinerals. Observing 



The Large Eirthqiinkes of 1905. 

 Earthquake districts .are indicated A, B, C, &c 



nd the 



.Shide, Isle of Wight. In view of the tragic interest of 

 the recent earthquake at \'alparaiso, we reproduce this 

 year's chart. Its details and its figures largely explain 

 themselves, and it is only necessary to add in further 

 explanation of them that the major earthquakes, such 

 as arouse disturbances large enough to be recorded on 

 the instruments in the larger number of the world's ob- 

 servatories, are recorded in large numerals, such as 

 A32. The letters A, B, &c., refer merely to the earth- 

 quake areas, Andean, Cordillerean, Himalayan, Antil- 

 lean, iVc. It w-ill be noted, as any intelligent observer 

 would have suspected, that the larger number of earth- 

 (juakc areas are situated where there is a great range of 

 mountains, and preferably where the mountain range is 

 in proximity to a sea-board, so that the mountainous 

 range slopes beyond the coast to the ocean floor. In 

 such a case the actual perpendicular distance from the 



Among these causes tidal influences and synchronous 

 solar disturbances have been suggested, but the most 

 interesting suggestion made during the last few- years 

 has been one which relates to the figure of the earth, 

 and to the movement of the earth's axis. The move- 

 ments of the earth's axis are not uniform, and if a 

 curve be plotted showing the path de.scrilx-d by the 

 earth's pole in its periodic cycle, it will be found that the 

 path traced is not regular, but contains irregular and 

 sudden alterations of the curve. Now if the earth were 

 a spherical body all parts of the surafce which were in 

 the same state of strain, it is conceivable that move- 

 ments of this nature would not affect the stability of its 

 crust. But it is evident that the crust of the earth is 

 not of uniform stabilitj-, and it has lately been suggested 

 by .Sir (1. H. Darwin and Mr. Jeans, of Cambridge, 

 that the earth is not sphere-shaped, but shows traces in 



