COMMENT AND CRITICISM. Thk recKEASEU favor with which the oio- genic theory of earthquakes — the theory that r^ards earthquakes as the effect of diaturb- ances due to mountain growth — has been looked upon in recent years must be aceouiit«d a distinct gain for physical geology. The vol- canic theory, now rationally liiiiited, has long lieen more popular. It is not long since Mallet, who has been widely (juoted as an authority on the question, committed himself to the narrow statement that "an earthquake in a non-volcanic region may, in fact, be viewed as an uncompleted elTort to establish a volcano. ' ' although he afterwards held a broader opinion. Lyell wrote in the last edition of his 'Principles' (1876), very much as in his first (1830). that "the principal causes of the vol- cano and the earthquake arc to a great extent the same, and connectetl with tlie development of heat and chemical action at various depths in the interior of the globe." More lately, Dau- br^e maintains a similar view, even after refer- ring to the suggestions of Dana. Suesa, and Iteim, and concludes that ■• earthquakes seem lo be like stifled eruptions which do not find ^^u outlet, about as Dolomieu thought."
^^Khie of the chief reasons for exaggerating ■VR value of the volcanic to the neglect of the orc^enic theory has been the improper reading of earthquake maps. The map constructed by Mallet in 1«S8, still the beat of its kinil, is very commonly quoted as showing a general agreement in the distribution of volcanoes and earthquakes; but it is quite unwarrantable to include the well-shaken regions of Spain or the Aljw, for esaraple, in the volcanic district of the Mofliterranean. The shocks of deraon- stnibly volcanic origin seldom extend far from their centres : the eruptions of Italy do not
El the adjacent countries. In the Alps io.lOD. — IMu.
��tbemaelves there is now no volcanic action whatever, nor has there been any of significant extent at any time in their geological history, so far as it is known. It is altogether gratuitous to suppose that the frequent tremors felt there I'esult fi-om concealed volcanic explosions ; for they find sufficient explanation in the forces that have made the mountains, which are un- doubte<Ily still growing.
Another cause for the former ueglect of the otogenic tlieory was the almost universal belief that mountain ranges had been lilted up or burst out by expansive force from beneath, instead of squeezed and crushed together by lateral compression, aa is now widely accepted. The difference has been concisely expi-essed bj- Stur of Vienna : formerly it was ' gebirgshub ; ' now it is 'gebirgsschub.' Of course, as long as geologists were generally of the mind that mountains were prodnced by uplift fVora be- neath, it was natural to associate surface shocks with smothered volcanic action, whether eruptions followed or not; but, with the disap- |)earance of the idea of uplift as applied to mountain ranges, it is as natural to refer earth- tremors in non-volcanic mountain icgions to the crushing forces that produce the disor- dered mountain structure. There is, indeed, now sometimes seen a disposition to gu, |)cr- haps, too far in this reaction, and exclude vol- canic action from nearly all share in causing earthquakes. Some of the English observers in Japan, a volcanic region par excellence, arc of this mind, and attribute the numerous smiill shocks, even there, to ntiuctural and not to volcanic disturbance. It is a difficult matter to decide. Indeed, the study of earthquakes must, in great part, long remain in a two- thirds condition. Oliservntions are plentiful. hypotheses have never been lacking ; bill veri- fication can hardly ever be attained.
The lack ov final and convincing verification
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