EARTHQUAKES. 743 



registering tide-guages of the Coast Survey, along the coast of Oregon 

 and California ; and, from the data thus afforded, he was enabled to 

 calculate the mean depth of the intervening ocean, stated on page 12. 



3. Cause of Earthquakes. — (1.) The chief cause is the lateral pres- 

 sure in the earth's crust, or conditions produced by it. Fractures, 

 cvushings, shovings, and minor displacements of the crust, or of its 

 overlying strata, have made the greater earthquakes of the past ; and 

 the same cause is probably the chief source of modern earthquakes. 

 The rocks have been everywhere left in a state of strain, in conse- 

 quence of the upliftings and foldings to which they have been sub- 

 jected ; and any yielding, however slight, is necessarily attended by 

 an earthquake shock. Professor W. H. Niles states that, at a quarry 

 of gneiss near Monson, Mass., bendings, sudden fractures, and expan- 

 sions of the rock 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 transverse to the strike. This may be an ex- 

 ample, on a small scale, of the strain that pervades the whole crust. 



All are familiar with the cracking 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 decline. In each case, there is a strain or tension ac- 

 cumulating for a while from contraction 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 

 subterranean undermining of strata ; the sudden evolution of vapors 

 about volcanoes ; and local changes of temperature in the crust. 



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

 chief cause of earthquakes. Investigations carried out 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. But, if the earth is not mostly liquid within, some 

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

 to be found. 



The earth's contraction from cooling is thus at the foundation of its 

 profoundest movements. But all these movements were only steps in 

 the grand system of evolution which was in progress. 



