Earthquakes.

Waikato Times, Volume XXIV, Issue 1985, 28 March 1885, Page 2 (Supplement)

Earthquakes.

A Si Tr\TMic Hi mi i th\t thia ari: C\i>ld li\ Sui.MDhNIA, NOT E\ri;O->ION. Pkoi\ Samuel K\n:r.\M), A. M., M. D, of Boston, sayq, in a lecture on earthquakes : The old theories of the causes of volcanoes have gradually lost the confidence of geologist?. Mallet's theory is now generally admitted as the beat explanation, to wit, that, in cooling from a highly heated nebulous matter, the crust of the earth has shrunk upon the centre, more readily according to its thinness, and the most so, therefore, in former geological ages : this shrinking and falling in of strata of immense extent and thickness, by the motion, develop a great amount of heat, sufficient to melt rocks and sedimentary deposits between the crust and the nucleus, which, by the agency of steam from admitted waters. are brought to the surface in the form of lavas. That the same cause has produced both the elevations and depression, on the earth's surface may also be inferred from the fact that the height of the loftiest mountains and the depth of tho deepest ocean are about alike. As connected with the cause of earthquakes here maintained, it may be mentioned that the 1794 eruption of Vesuvius yielded 46,000,000 cubic feet of lava, that of Etna, in 1669, twice that amount, and that of Skaptar Jokul, in Iceland, in two years, twenty-one cubic miles. Where the heat generated by fiction of subsidence is not sufficient to liquefy the rockß, mud volcanoes, as in Java and Hawaii, are produced ; and as the volcanic energy lessens, geysers and hot springs denote the expiring forces, Though in such localities you may expect the earthquake, as a rule you need not fear an outburst. The theories of earthquakes are numerous and fanciful in the extreme. No satisfactory connection with atmospheric conditions has been proved, except as a mere accessory ; they occur everywhere, at all seasons of the year and in all geological formations and times. They seem to follow no laws of periodicity, and are apparently independent of volcanoes. The theory of their nature usually accepted now is that of Mallet, viz , that an earthquake is the passage of a wave past an observer, in any direction, with a force and velocity depending on the activity of the cause and the nature of the strata through which it passes — a vibratory movement, like that of a wave of sound, transmitted, reflected and refracted by the earth ; it may originate beneath the sea, causing a destructive tidal wave hundreds of miles distant. This wave can be measured by a cunningly devised instrument. Having vmited the earthquake-rpnt districts of Iceland, the Sandwich and Philippine Islands and the Mediterranean basin. I am of the opinion that almost all extended modern earthquakes have been the result of subsidence, and not of explosions. Wherever, in the cooling, shrinking crust, support is taken away by contraction, pressure, chemical action, emission of lavas, corroding and heated waters, occasional explosions, aided possibly by terrestrial, meteorological or astronomical causes, the strata fall in ; these may be hundreds of miles in extent and leagues in thickness, with more or leas violent depression, producing a shock which may be transmitted to great distances by land or beneath the water. In certain fractures of the crust, produced even in remote geological ages and in favourable strata, much heat and consequent chemical action may liquefy the surrounding rooks and cause a flow of lava. This undermining or corrosion I conceive to bf the predisposing cause of earthquake 0 ti 1 i v being tho effect carried for long di Ilarieeg, with a violence and direction depenui,ia, on the elastic character of the strata. In old geological times the dislocations oi up lifts of strata, called " faults," were doubtless accompanied by earthquakes and were then their moat common cauae ; but, as the tension and pressure which formed the then immense folding have in great measure ceased, subsidence may be a more satisfactory explanation than upheaval.