138 POPULAR SCIENCE MONTHLY 



nection with the fracture system of the more firmly consolidated 

 rocks. 



Either the same or a separate triangulation section of the party 

 should have charge of the re-occupation of primary triangulation sta- 

 tions in order to see what changes in position and elevation of these sta- 

 tions are properly ascribable to the earthquake. It may well be doubted 

 if more ideal conditions could anywhere be found for such a study. 

 If continued changes should be found to occur during the progress of 

 the surveys, as is by no means improbable, the opportunity thus offered 

 to compare mass movements of the ground with the time of prominent 

 aftershocks should be regarded as of the first importance. 



In every great earthquake which is studied, perhaps the most 

 important line of attack is found in the distribution of the surface in- 

 tensity of the shocks. It is now everywhere acknowledged that this 

 intensity or amplitude of movement (and it is on this that damage 

 to structures depends), is in a large measure determined by the elastic 

 or non-elastic nature of the underlying material. Amplitude of move- 

 ment is least on so-called " solid rock," it is greater on non-coherent 

 deposits such as alluvial material, and it is probably greatest over so- 

 called " made ground," with its tin-can and crockery ingredients. 



With the passing of the centrum theory it is inevitable that the 

 study of the immediate basement of each locality should enter ujDon the 

 quantitative stage of development. The local quantitative effect of 

 the surface layers is a factor which to an approximation may be kno^vn 

 and for this reason should be eliminated, if the seats of movement are 

 to be determined. Local thickness and relative elasticity of the uncon- 

 solidated materials in the basement must therefore, be determined, and 

 the value thus obtained be deducted from the total local intensity, if we 

 are to arrive at the genesis of the disturbance. Accurate geologic maps 

 and earlier detailed seismological studies in Calabria and Sicily are fav- 

 orable to an extended study of this subject.^ 



There are few, if any, places where within a circumscribed area 

 more elaborate magnetic observations have been carried out than about 

 the straits of Messina. Before the earthquake of 1905 a detailed mag- 

 netic survey for this district had been completed. It is almost certain 

 that large changes would be revealed by a new survey since it has been 

 shown in Japan that important changes in the isomagnetics resulted 

 from the great earthquake of 1891. The importance of magnetic 

 records to earthquake study is each year being made more apparent. 



* William H. Hobbs, I., " On Some Principles of Seismic Geology," with an 

 introduction by Professor Eduard Suess ; II., " The G^eotectonic and Geodynamic 

 Aspects of Calabria and Northeastern Sicily," with an introduction by Count 

 de Montessus de Ballore. Gerland's "Beitraege zur Geophysik," Vol. 8, 1907, 

 pp. 219-362, pis. 12. 



