158 Charles Davison — Earthquake Phenomena. 



any area being often greatly in excess of the number of faults 

 beneath that area. Hence, it follows that the majority of non-volcanic 

 earthquakes must be due rather to the individual slips which are 

 the elements of a great displacement than to the repeated Assuring 

 of the earth's crust. On this view, then, the earthquake is but an 

 incident in terrestrial evolution, and has no further connexion with 

 the formation of mountain-chains and continents than the creaking 

 of an unoiled wheel with the motion of a railway train. 



"First, let us take the case of an earth-crust homogeneous and of 

 equal elasticity in all directions. Let EF represent a section of 

 a fault or fissure, and C D the sur- 

 face of the earth, and let us call 2? 

 the rock-masses on either side the G — D 

 fissure A and B. Now suppose 

 B to be slightly, but suddenly, B 

 lowered relatively to A. Then the 

 particles of A at the surface of 

 the fissure will by impulsive fric- 

 tion be drawn sharply downwards. 

 Similarly, those of B at the surface 

 of the fissure will be drawn up- ^ 

 wards. Hence, the earth-waves in 



the two rock-masses will start in opposite phases of vibration. At 

 a distance from the fault this difference will not much affect the 

 character of the earthquake-motion ; but along the line of fault, 

 every particle, being urged upwards and downwards equally, will 

 remain at rest; and in the immediate neighbourhood of the line 

 of fault, or its continuation, the disturbance will be less violent, 

 than it otherwise would be, owing to partial interference by the 

 spreading of either earth- wave in the adjoining rock-mass. 



Eeturning to the case of the earth-crust as it actually is, hetero- 

 geneous and discontinuous in all directions, we find the problem 

 somewhat less simple. We have here to guide us the numei-ous 

 observations of earthquakes in Japan and other places, and, to a 

 certain extent, the seismic experiments of Prof. Milne and Mr. 

 Gray. These lead us to infer that absolute rest along a line of 

 fault must be a most improbable occurrence, owing to the great 

 irregularity of the surface rocks. Still, we may believe that, in 

 many earthquakes, the disturbances will be less violent along, and 

 in the neighbourhood of, the line of fault, than in other parts of 

 the disturbed area ; and that in earthquakes of moderate intensity, 

 the motion may be so slight as to be imperceptible to observers 

 unpractised or without instrumental assistance. Evidently, also, 

 the further away a place is from the spot where the fault-slip 

 occurs, the less likely will it be for the earthquake-waves to 

 interfere there. 



The Scottish earthquake of November 28, 1880, furnishes a good 

 example of these considerations. It occurred at 540 p.m., after 

 the hour of sunset, and therefore at a time when all the lighthouse- 

 keepers must have been on watch. Mr. C. A. Stevenson, who has 



