GEOLOGY. 309 



the vent, if any, may be at a great distance from the immediate source of 

 disturbance. 



Judging from their great frequency, from their constant recurrence in cer- 

 tain districts, and from the comparative periodicity of extreme paroxysms, 

 as well as of minor convulsions, there would seem little doubt that some 

 widely-spread general cause, cosmical, and not merely terrestrial, is con- 

 nected with and governs the forces brought into play on these occasions. 

 From what we know of the electric force and its relations with earth-magnet- 

 ism, of the approximately superficial character of earth-magnetism, and of 

 the certainly small depth of most earthquake movements, as proved by the 

 small area they aifect, we can hardly avoid the conclusion that to the agency 

 of heat, initiated by electricity, and connected with those chemical results 

 produced by the mutual action of natural substances under certain conditions 

 of contact, we are indebted for the paroxysmal movements which record at 

 the surface, however obscurely, what is going on far away out of our sight 

 in nature's great laboratory. 



Under the influence of the attraction of the moon and sun, the portions of 

 the interior of the earth in a fluid state, no less than the w r aters of the ocean 

 and the gases of the atmosphere, are doubtless subject to tidal influences; 

 whilst the direct but periodic action of the rays proceeding from the sun, chang- 

 ing, as it appears they do, during long but definite cycles, and according to 

 laws which we seem now only beginning to recognize, may, and most proba- 

 bly does, bring about periodic paroxysmal action Avithin the earth, which 

 may thus seem to depend on months or seasons, on the moon's position, or 

 the sun's clearness or obscurity, or on the magnetic state of the higher parts 

 of the atmosphere, causes which one would think quite unlikely to have 

 influence on what goes on so far out of the immediate range of their action. 

 It is true that nothing has yet proved a direct connection between earth- 

 quakes and such cosmical phenomena; but what is now known supports tho 

 conclusion, that changes in terrestrial temperature, and in the circulation of 

 electric, thermic, or magnetic currents, converting heat into force, may pro- 

 duce, or in some way govern, the chemical changes which result in an earth- 

 quake. 



No doubt both sun and moon do, either directly or indirectly, by light, by 

 heat, and by electricity, as well as by the force of gravitation, largely influ- 

 ence all that is above, upon, or beneath the surface of our planet. Perhaps 

 also it w r ill be found some day that the nature, and even the extent, of this 

 influence are within the range of human discovery. These are, however, yet 

 among the dark and unexplained mysteries of nature. We may suggest 

 mutual relations, but we cannot follow out these suggestions into practical 

 and definite conclusions. 



And, lastly, what is our own position with reference to the chances of 

 earthquake disturbance? We live in an area certainly subject to such move- 

 ments, and not very far from those countries w r here the most severe earth- 

 quakes on record have happened. Earthquakes have frightened our fore- 

 fathers, and may overwhelm us. The fatal explosion may happen this or 

 next year; it may not happen in this century. It may originate beneath our 

 very feet, or at the bottom of the ocean near our shores ; or it may take 

 place so far away that we hear only the faint, distant echoes of the convulsive 

 throe; but we are not the less cei'tainly living over a mine ready to be sprung, 

 and no one can tell when or where the fatal match will be applied. 



