1906-7. TRANSACTIONS. 131 



to carr}^ away so many tons or cubic miles of limestone rock? 

 The result of our investigation is a hole in the ground, or rather a 

 hole in the earth, and if we make that hole big enough, something 

 is going to happen — the roof is going to collapse — and we have an 

 earthquake. This in brief is the Einsturz or downthrow theory. 

 Now some earthquakes have happened which might be explained 

 by the above, but, for, by far the larger number, other reasons 

 must be sought. Before dismissing the above, it may be sug- 

 gested, that, although subterranean waters must and do hollow 

 out the earth's crust, the formations slowly adjust themselves to 

 the minute changes continuously wrought by the action of water, 

 so that excessively large cavities are improbable if not impossible. 



One of the oldest beliefs about earthquakes is their intimate 

 associations with volcanoes. This opinion died a hard death, in 

 fact, I am not sure that it is quite dead yet, however Professor 

 Milne has shown, especially for Japan — the principal earthquake 

 country — -"that the many quakes of that archipelago seemed to 

 show an avoidance of the volcanic centres which are numerous 

 in the interior and to indicate that volcanic energy was seldom 

 concerned in generating them. " 



Volcanoes are as a rule shallow-seated, while the movements 

 of earthquakes are tectonic, that is, affecting large areas of the 

 earth's crust and miles in depth. 



Another theory that was advanced some forty years ago 

 was the tidal theory. Assuming the earth to be a molten mass 

 covered by only a thin shell, it seemed plausible to have tidal 

 effects by the attraction of the moon and of the sun. Spring and 

 neap tides in the ocean would be manifested by earthquakes of 

 greater and less intensity; similarly for perigee and apogee. 

 However, facts don't fit in right, and the fortress has been aban- 

 doned. 



When we say that earthquakes are the result of the adjust- 

 ment of strains and stresses within the earth, the statement is one 

 practically accepted by all seismologists, at the same time, it is 

 far from explaining the cause. What sets up these strains and 

 stresses, and if there are various agencies at work, what is their 

 relative effect? 



It is almost axiomatic to assume the earth as a cooling body. 

 Now on this assumption a very pretty scientific theory was ad- 

 vanced some thirt}^ years ago by Lowthian Green, it was the 

 tetrahedral theorv of the figure of the earth. Let us follow it for 



