198 



SCIENCE 



[Vol. V., No. 109. 



between the disturbed areas and the structural fea- 

 tures of the Alps will be looked for with interest. 



W. M. Davis. 



THE CAUSES OF EARTHQUAKES. 1 



I have followed with much interest the details upon 

 the recent earthquakes, which the newspapers have 

 published ; but this question is so intricate, so difficult, 

 that I assure you I should not have undertaken its 

 investigation had I thought any other person would 

 have been willing to do so. Meanwhile, at the acad- 

 emy, the question is growing in importance, geolo- 

 gists, geodesists, and others having taken it up with 

 considerable enthusiasm. Under these conditions, I 

 have thought that I ought not to draw back. Never- 

 theless, I am not without a certain apprehension. 

 Indeed, the question of earthquakes is one of the 

 vaguest. Data are hitherto wanting, but there is no 

 lack of theories; for as in medicine, when there are 

 many remedies for one disease, it is frequently the 

 case that neither is really good, so in geology, in ter- 

 restrial physics, when many theories are put forward 

 to explain a phenomenon, it is necessary to cast aside 

 each, and say that none is absolutely sufficient. I 

 start, then, with a certain hesitation ; and yet, when 

 one accepts an appointment to study facts of this 

 sort, it seems to me necessary to have in mind some 

 theory, true or false, and to adopt it more or less 

 boldly, free to abandon it after contradiction. 



I start, then, with a certain idea which I expect 

 to verify or invalidate. I do not propose to tell you 

 what it is: I will simply ask your permission, before 

 giving my plan of studies, to point out in a few words 

 the current theories to account for earthquakes. 

 . There are four principal ones. They are very old. 

 We find them in the Greek authors, and perhaps, if 

 one were to search carefully, they would be found 

 among East-Indian traditions. The first is based 

 upon the supposition, that, under the solid crust of 

 the earth, the sudden generation of gases and vapors 

 causes subterranean explosions; and it is the effect of 

 these shocks that we feel on the surface. This would 

 be in a way comparable to an explosion of dynamite 

 taking place at a great depth. I need not discuss 

 these theories, yet I may say that perhaps this one is 

 true when applied to earthquakes in the neighborhood 

 of volcanoes. It is certain, indeed, that as soon as 

 the earth opens, great quantities of gas are liberated 

 from beneath the surface, where in some way they 

 have been generated and furnished with extraordi- 

 nary power. 



But even if this theory is probable with regard to 

 volcanic earthquakes. I think that it would be dif- 

 ficult to apply it to those in Spain. 



A second theory has been proposed by a learned 

 physicist, Alexis Perrey. It is based upon the sup- 

 position that the combined influence of the sun 



1 A communication to the French geographical society, on 

 Jan. 23, by Mr. FouquG, professor of geology in the College de 

 France, and chief of the commission appointed by the Academy 

 of sciences to study the Spanish earthquakes. 



and moon, acting upon the liquid parts beneath the 

 surface, produces tides analogous to those on the sur- 

 face of the earth. These vast tides of liquid fire at 

 certain favorable movements, striking upon the solid 

 external crust, cause the earthquake shocks. I also 

 abandon this theory, for I do not think it can apply 

 to Spain. 



There remain two others, one that of Scheuchzer, 

 a distinguished savant, at once paleontologist, geolo- 

 gist, and physicist. Having studied the earthquakes 

 in Switzerland, he has attributed them, not without 

 reason, in certain particular cases, to the falling-in 

 of subterranean caverns caused by the dissolving-out 

 of such substances as salt or gypsum by water which 

 has penetrated beneath the surface. Such a collapse 

 would, without doubt, cause a very appreciable shock 

 at the surface of the earth. This theory may apply 

 to certain special cases; but it remains to be seen if 

 it applies to the Spanish earthquakes. 



There is a fourth which is at present in favor in 

 Germany among nearly all geologists of that country, 

 and it has also been accepted by some in other coun- 

 tries. In France it has not been so well received : 

 nevertheless, there are eminent men who entertain it. 

 It is based upon geological observations. There are 

 no geologists, indeed, who, observing the walls of the 

 cracks in the metamorphic rocks, for instance, have 

 not been struck by the fact that these beds, originally 

 deposited in a horizontal position, have been raised 

 and broken. There have evidently been movements 

 of extreme importance, since rocks that were origi- 

 nally connected and regular are now in the greatest 

 disorder. Now, it is certain that these movements 

 could not have been produced without superficial 

 shocks at the moment when the fissures were made. 

 Therefore there must have been earthquakes in all 

 geological epochs, even the most ancient, which are 

 exactly comparable with those of to-day. But recip- 

 rocally, if these ancient foldings have produced earth- 

 quakes, why are not the present earthquakes the result 

 of analogous phenomena ? 



You see that the theory is perfectly regular up to 

 this point. It is only necessary to know (the diffi- 

 culty is merely thrown back in time) what is the 

 origin of these foldings, of these fractures. Why 

 these out-throws, these subsidences, these convolu- 

 tions? We then arrive at a very old explanation, 

 given by geologists, and still admitted by many 

 savants. It is that the earth is continually cooling, 

 and so contracting. The superficial crust has reached 

 a nearly constant temperature; but this is not true 

 of the liquid portions adjacent to it, where the tem- 

 perature must be very high, though constantly cool- 

 ing. In cooling, its volume becomes less, and its 

 contractions cause foldings and fractures in the solid 

 crust. This theory is rather old, it is true, but there 

 is no better theory at present. 



As to the Spanish earthquakes, it seems to me, 

 that, of these four theories, only two should receive 

 any attention. 



The question is, therefore, whether there are fis- 

 sures, bendings, and faults beneath the surface, or 

 whether the water is dissolving out caverns. In a. 



