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DYNAMICAL GEOLOGY. 



3. Earthquakes. 



An earthquake is a vibration within the rocky sides of the earth, 

 begun in some deep-seated region of local disturbance, and propagated 

 upward and outward from that place as a centre. The vibration is a 

 longitudinal wave, like the sound-wave in the atmosphere, but it is 

 varied in its movement by differences in the elasticity of the rocks 

 traversed, and often also by their want of continuity. The disturb- 

 ances probably come, for the most part, from yielding to a pressure 

 caused by the previous conditions connected with mountain-making in 

 that part of the sphere. They are most likely to be located where 

 there was previous weakness or a course of former fracture. Regions 

 of volcanoes, extinct or active, are also regions of earthquakes ; and 

 as these are situated mostly along the borders of the continents, or in 

 and near the great transverse seas dividing the northern and southern 

 continents, so these are the areas that suffer most from shakings. 



The observations of Professor W. H. Niles on the gneiss of a quarry at Monson 

 Mass., show that even the solid rocks are in some places under a strain; for he 

 states that bendings, sudden fractures, and expansions of the rock often take place ; 

 masses, before their ends are detached, become bent upward at middle ; and one mass, 

 three hundred and fifty-four feet long, eleven wide, and three thick, was an inch and a 

 half longer after it was detached than before; showing a strain which was greatest in 

 a direction from north to south — an effect due to compression by the pressure the rocks 

 had been subjected to and a consequent expanding in a transverse direction. All are 

 familiar with the crackling sounds occurring at intervals in a board floor of a house, and 

 arising from change of temperature, especially in a room in winter that is heated only 

 during the day; and with the more common sounds of similar character from the jointed 

 metallic pipe of a stove or furnace, given out after a fire is first made, or during its de- 

 cline. In each case, there is a pressure or tension, accumulating for a while from con- 

 traction or expansion, which relieves itself, finally, by a movement or slip at some 

 point, though too slight a one to be perceived; and the action and effects are quite 

 analogous to those connected with the lighter kind of earthquakes. 



There are other causes for local shakings, among which are — the undermining of 

 strata; the sudden evolution of vapors about volcanoes; and local changes of tempera- 

 ture in the crust ; but these are of minor importance. 



Tidal waves in the internal igneous material of the globe have been considered a 

 chief cause of earthquakes. Investigations carried on by Alexis Perrey, of Dijon, 

 France, have seemed to indicate that there is a periodicity in earthquakes, synchronous 

 with that in the tides of the ocean, — the greatest number occurring at the season of 

 the syzygies, in each lunar month. If the earth has not very free liquidity within, 

 some other explanation of the facts, so laboriously worked out by Professor Perrey, 

 will have to be found. 



The sudden fracture or displacement causing an earthquake pro- 

 duces a small longitudinal oscillation in the rock particles, the ampli- 

 tude of which may not exceed a few inches, or even one inch. But 

 the wave-motion thus engendered travels to great distances ; and it 

 has a velocity generally of about 20 miles a minute, while that of the 

 oscillating rock-molecules is very much less. 



