between IhedUliirbed areas and the structural fea- lurea o( Ihe Alps will be looked for with interest.
W, M. Davis.
���ivoi, v.. No. m.
��THE CAUSES OF EARTHQUAKES.*
E followeilwith much Interest the details upou nt earthqualtes, which the newspapers liave pubKehed ; hut this ijnestion U so intricate, so difficult, that I assure you I should not have underlaiten its investigation had I thought any other person would have heen willing to do so. Meanwhile, at llie acad- emy, the question is growing in icnportance, geolo- glils, geodesista, and others having taken It up with considerable enthusiasm. Under these conditions, I have thought that I ought not to draw l>acfc, Never- theless, I am not without a cerlain apprehension. Indeed, the question of earth <|ualies is one of the vaguest. Data are hitherto wanting, but there iti no tack of theories; for at in medicine, when there are many remedies for one disease, it is fretjueQlly 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 east aside each, and say that none Is absolutely sufficient. I start, Uii;n, with a cert^n hesitation; and yet, when one accepts an appointment to study facts of this sort, it searaa 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 slart, then, with a certain idea which I expect to verity or invalidate, I do not propose to tell you what It Is; I will simply asit your permission, before giving my plan of studies, to point out in a few words the current theories to account for eanhquakes.
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 Eastrlndian traditions. The first is based upon the supposition, that, under the solid crust of the earth, the sudden generation of gases and vapors cause* 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 liene^th Uie surface, where in some way they have been generated and furnished with extraordi- nary power.
But even if this Iheory is probable with regard to volcanic earthqualies. 1 think that it would be dlf- flcolt 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 snn
��and moon, acting upon the liquid parts beneath Uk surface, produces tides analogous to those on the iur- face of the earth. These vast tides of liquid fire at certain favorable movements, striking upon the saIM external crust, cause the earthquake shocks. I also abandon Ibis theory, for I do not think it can apply to Spain,
There remain two others, one that of .Scheuchrer, a distinguished »avan(, at once paleontologist, geolo- gist, and physicist. Having studied the earthquakes in Switzerland, he has attributed Ihem, 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 lias penetrated beneath the surface. Such a collapse would, without doubt, cause avery appreciable shock at the surface of the earth. This Iheory may appii to certain sgieciai 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 whoentertain IL It is based upon geological observations. There are no geologists, indeed, who, observing the walls of the eraeka in the metamorphic rocks, (or instance, hare nut been struck by the fact that these beds, originally deposited in a horizontal position, have tieeii raised and broken. There have evidently been movements of extreme importance, since rocks that were origi- nally connected and regular are now tn the greatest disorder. Now, it is certain that these movemenO could not have been produced without superficial shocks at the moment when the fissures were made. Therefore there must have been earthquakes in aU geological epochs, even the most ancient, which an exactly comparable with those of to-day. But recip- rocally, If theae ancient foldings have produced ei ""^ quakes, why arc not the present earthquakes the rt of analogous phenomena?
Ton see that the theory is perfectly regular this point. It is only necessary to know (the d culty is merely thrown back in time) what ii origin of these foldings, of these fractures. Why these out-throws, these sutyaidcnecs, these convolu' tions? We then arrive at a very old explanation, given by geologists, and »tiI1 admitted by many ntcants. It is that the earth is continually cooling, and so contracting. The superficial crust has reached a nearly constant temperature; but this is r of the liquid portions adjacent to it, where the b perature must be very high, though constantly o ing. In cooling, its volume becomes less, and 1 contractions cause foldings and fractures in the s< crust. This theory is rather old, it is true, but 11 is no l)etter theory at present.
As to the Spanish earthquakes that, of these four theories, only two should r
The question is, therefore, whether there a sures, bendings, and faults beneath llie surface, i whether the water is dissolving t
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