Ixviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [vol. Ixxviii. 



The familiar lecture experiments of supersaturated solutions, which 

 remain liquid until some disturbance, or the introduction of foreign 

 matter, causes a rapid solidification, accompanied by a change of 

 bulk, offer another analogy ; and a third group of possible changes 

 is represented by those allotropic alterations with which we are 

 familiar, of which the alteration of aragonite into calcite may 

 be quoted as an example. 



It is not improbable that, in the material beneath the outer 

 crust, changes, more or less analogous to one or other of these 

 t3'^pes, are taking place, some slow and gradual, others more rapid 

 and sudden, but all accompanied by a greater or less change of 

 bulk, either of increase or decrease ; and, if this be accepted, we find 

 an explanation, not only of the forms and origin of earthquakes, 

 but of many other phenomena, which are difficult of explanation on 

 any hypothesis of contraction and compression alone. On the one 

 hand, slow movements of elevation such as that of the northern 

 Scandinavian region may be attributed to slow and gradual change 

 involving the whole bulk of large masses, the lesser earthquakes 

 may be due to more rapid changes in smaller portions, the greater 

 to transformations involving a larger bulk of material, and possibly 

 a more abrupt change of combination and densit}^ ; while the 

 greatest earthquakes, of first-class magnitude, result from similar 

 changes involving a large bulk of material, the difference between 

 the origin of small and great earthquakes being analogous to the 

 difference in the effect of the explosion in a shot-gun, and that of 

 the Vimy mines, or the recent havoc at Oppau. 



To elaborate these considerations forms no part of my purpose ; 

 enough has been said to show that, even in our very fragmentary 

 knowledge of what goes on within the substance of the Earth, we 

 have means of explaining and interpreting the greater part of the 

 facts known to us regarding the character of earthquakes. I shall, 

 therefore, end my address by summing up the conclusions which 

 have been put forward, as to origin and cause. These are, first, 

 that earthquakes are not due to any slow acting process of secular 

 duration, but to a rapid, possibly instantaneous, development of 

 strain, a conclusion which I believe to be true of the greatei' part, 

 at least, of those earthquakes usually classed as tectonic, and of 

 all those of great magnitude ; and, secondly, that the development 

 of strain is not the result of pi'ocesses which have produced the 

 tectonic structures recognized by surface observation, but to 

 chano-es and displacements in the matter which lies below the 

 cooled and solid outer crust. 



