652 M. F. DE MONTESSTTS DE BALLOBE ON [Nov. 1 8 96, 



III. Description of the Globe from a Seismic Point of Yiew. 



The earthquake-catalogues inform us that in such and such a 

 locality so many shocks have heen reported. Wow it very often 

 happens that this locality is not the focal centre of the phenomenon. 

 But earth-shocks in general have a smaller 'area of activity' than 

 one might imagine from the narratives of great earthquakes. It 

 has been demonstrated that this area, as a general rule, covers a 

 surface far smaller than that described by a circle of 12| miles 

 radius. Consequently the smaller earthquakes, such as are known 

 to have shaken only one locality — and these are by far the most 

 numerous, — are sufficient for showing upon the map the distribution 

 and density of the centres of seismic movement ; for, if we do not 

 as a rule know which is the real centre, at least we know a point 

 which is usually much nearer to it than 12| miles. By grouping 

 these localities or centres one is enabled to define on the map 

 regions of greater or less stability. It is well, for the sake of giving 

 these rational boundaries, to make use of the chief topographical 

 features of the country — mountains, rivers, seas, etc. 



It remains now to find a numerical value for the greater or less 

 stability of a region, what we may in a word term its ' seismicity.' 

 Mallet did by implication trench upon the problem (Fourth Eeport 

 upon the Tacts and Theory of Earthquake-phenomena, Proc. Brit. 

 Assoc. 1858). He drew out a map of the world whereon graduated 

 shading represented the frequency and intensity of earthquakes in 

 various parts of the globe. Unfortunately he had to base his work 

 on estimates, in the absence of actual data. The result was that he 

 often fell into serious error — as, for example, with regard to the 

 Antarctic region, a very stable area. Moreover, that deeply learned 

 seismologist appears to have confounded seismicity with vulcanicity 

 — two entirely different and independent factors, pace the usually 

 received opinions. 



O'Reilly (Trans. Roy. Irish Acad. 1884) published a seismical 

 chart of Great Britain, and it is a matter for deep regret that the 

 similar chart of Europe (foreshadowed in his highly important 

 alphabetical catalogue of earthquakes in that part of the globe 

 and adjacent regions, op. cit. 1886) was never published. These 

 documents, though extremely interesting, are difficult to utilize 

 because they record every shock felt in each locality indicated, 

 without any attempt on the author's part to show whether these 

 localities were centres of vibration or were at all near the centre. 



The seismicity of a given area evidently depends upon the number 

 and intensity of the shocks felt within that area. The first factor 

 may be easily tabled, but one cannot say the same of the second. 

 Yery fortunately we are enabled to eliminate it for two reasons : 

 the number and intensity of earthquakes are practically proportional 

 in any particular country — by which it is meant that they are 

 severe only where they are frequent, and conversely — provided that 

 we take into account a sufficiently long lapse of time. Moreover, 

 feeble shocks being enormously more frequent than severe shocks, 

 we may neglect the latter in the present investigation. 



