Page:The American Cyclopædia (1879) Volume VI.djvu/373

EARTHQUAKE 365 gions that are at present comparatively free from sensible earthquakes are Egypt and the eastern and southern portions of Africa, north- ern Europe and Asia, Australia, many portions of North America, the eastern portion of South America, and Greenland. Chronological Dis- tribution of Earthquakes. The contortions of geological strata, their faults and fissures, and still more the presence of old trap dikes, &c., that break through every known geological formation, sufficiently show that in every part of the world earthquake phenomena have been common at some time throughout the entire history of the globe since the formation of its solid crust ; indeed, we have in these phenome- na the record of the condition of the interior of the earth at successive stages separated by long intervals of time. The periodicity of earth- quakes has been studied by many writers, of whom among the earliest was Merian, whose conclusion, confirmed by Mallet, Perrey, Yol- ger, Kluge, Bridgman, &c., was that these oc- cur more frequently in the winter than in the summer months, as for instance in New Eng- land, Bridgman finds that between 1638 and 1869, of 227 earthquakes, 148 occurred during the winter half of the year, and 74 during the summer half. If this were an annual period independent of the wet and dry climatological seasons before alluded to, it would be of deep import ; but on the contrary it is less marked in some countries than in others, its laws being apparently peculiar to the various centres of shock ; it has therefore been treated in the pre- ceding section as one depending on climatologi- cal considerations. Mallet has indeed shown that, considering the whole world in one view, there is a preponderance of shocks recorded in certain months ; but it is possible, as he states, that this preponderance results from the great- er fulness of the records relating to them for the northern hemisphere. A daily period has also been recognized, such that more shocks are recorded during the day than during the night. This periodicity, which has been de- monstrated for Switzerland, if it (as thus enunciated) be a law of nature, will be so neu- tralized by the corresponding daily period of the antipodal regions, that in the total sum of the earthquakes over the whole earth no such period will be noticed. The gradual dim- inution and ultimate cessation of earthquakes, as maintained in 1771 by Castilhon, and the periods depending on hurricanes and on electric phenomena, are all too poorly established to be worthy of more than a passing mention. Thus also must we dismiss the attempts of Falb and others to predict earthquakes for any consider- able period in advance. The other periods of really cosmical importance are as follows: 1. That depending on the variations in the solar spots. 2. That depending on or connected with terrestrial magnetism. These two classes of phenomena, however, have as yet not been so well established as is necessary to their full acceptance. Lament (1840), Yarley (1870), and Sumichrast (1871) trace a connection be- tween terrestrial magnetism and earthquakes, though they differ as to whether the former is to be considered as the cause or the consequence of the latter ; most likely is it that both are consequences of a deeper common cause; in either case, however, there results a periodicity of earthquakes allied to the known periods in terrestrial magnetism. 3. The periods demon- strated by Perrey to be connected with the moon's phases. 4. That discovered by Palmi- eri for Mount Vesuvius, the violence of whose eruptions perceptibly increases twice in the so- lar day in a manner similar to the ebb and flow of the tide. Both these latter may possibly be actual laws, though the modes of their connec- tion are quite unknown and are likely to prove very complicated. 5. The quasi-periodical al- ternation of force announced by Mallet in his fourth report, such that a season of many se- vere shocks is followed by one of very few and sometimes slight shocks. 6. Mallet also de- tects indications of an increase in the frequency of earthquakes about the middle and end of each century. 7. The periodicity suggested by the facts first published by Humboldt, who remarked that in Mexico, South America, and the United States, the centres of earthquake influence seem to change their positions. Per- rey and Mallet, agreeing with Miraldi and with the ancient traditions of Italy, announce the same fact for that peninsula ; in it the numer- ous earthquake centres follow the general line of the ridges of the Apennines, as indeed Mal- let has shown to be the rule over the whole world; but further it is shown that in Italy each of these centres has apparently a slow secular movement along the general line, so that its greatest energy is not recurrent at the very same spot, or not until after a long period. Physical Changes produced ty Earthquakes. Permanent upheavals and depressions affecting the physical geography of large regions of the earth are considered as being on an extended scale manifestations of the same force that suddenly elevates and depresses smaller dis- tricts at the moment of an earthquake. Dana, Lyell, and others, presume that extended changes take place with extreme slowness and comparative quiet; and Prevost, Dana, Mallet, and others, attribute these to the influ- ence of tangential pressures and to the slow cooling and contracting of the earth. Beaumont and others ascribe the elevation of the great mountain chains to single upheaving shocks, with of course accompanying earthquakes of the greatest severity. The former views have now far more adherents, they being more in ac- cordance with the customary modes of opera- tion in nature ; it is however not to be denied that both methods of operation are admissible. Of sudden changes may be noticed the eleva- tions during the earthquake at Concepcion in 1835, and the depressions attending the shock at Lisbon in 1755. Of slow movements we find examples in the upheaval of the Scandina-