GEOLOGY. 277 



height from an inch or less to three feet, the latter being- its vertical 

 height in great earthquakes ; the length depends on elasticity of the 

 strata. The line where one quality of strata meets another is always 

 marked by the greatest havoc. JDolomieu states, "that in Calabria 

 the shocks were felt most formidably, and did most mischief, at the 

 line of junction of the deep diluvial plains with the slates and granite 

 of the mountains, and were felt more in the former than in the hard 

 granite of the latter. Houses were thrown down in all directions 

 along the junction, and fewest of anywhere these were situated in the 

 mountains. But if the case be converse, if the earth-wave pass from 

 highly elastic rock into a mass of clay or sand (suppose it lying in a 

 small-sized valley), and pass across this into similar elastic rock at 

 the opposite side, all the former results will follow." 



As before observed, great earthquakes originate beneath the sea; 

 those which have their focus inland are less destructive ; the shock is 

 generally lost beneath the ocean ; or, if a powerful one, it traverses 

 the bed, and is felt in distant countries. The movement of the Lisbon 

 shock was twenty miles in a minute, 1,750 feet per second; its 

 effects were felt in Scotland. " At Loch Lomond, the water, without 

 any apparent cause, rose against its banks, and then subsided below 

 its usual level : the greatest height of the swell was two feet four 

 inches. In this instance it seems most probable that the amplitude of 

 the earth-wave was so great, that the entire cavity or basin of the 

 lake was nearly at the same instant tilted or canted up, first at one 

 side, and then at the other, by the passage of the wave beneath it, so 

 as to disturb the level of the contained waters by a few inches, just as 

 one would cant up a bowl of water at one side by the hand." Many 

 experiments have been made by scientific observers to determine the 

 rate of motion through different materials. The velocity of wave- 

 transit through limestone (soft lias) is 3,640 feet per second ; sand- 

 stone, 5,248 ifeet ; limestone (primary marble), 6,696 feet; limestone 

 (hard carboniferous) , 7,075 feet ; and clay-slate, 12,757 feet. A glance 

 at these figures will enable us to conceive something of the consequen- 

 ces that must ensue when such immense rapidity is suddenly checked 

 or disturbed by meeting with strata of different elasticity. There is, 

 then, no difficulty in understanding why every thing on the surface 

 should be prostrated ; that frightful chasms should open, which, in 

 closing again, have actually bitten human beings in two. But it 

 must be remembered that, appalling as these convulsions may be, 

 they " do not properly constitute part of the earthquake at all ; and in 

 order to form a clear notion of earthquake mechanics, we must care- 

 fully distinguish between these, which are but consequences of the con- 

 sequences of the earthquake, and the earthquake-wave itself, which 

 gives rise to them all. The earth-wave shakes the country ; the fea- 

 tures of its surface are altered by the filling of valleys and levelling of 

 eminences ; a new state of things is instantly brought about as regards 

 its drainage ; and all its meteorological circumstances alter in propor- 

 tion. Hence, when, in the loose narratives of earthquakes, we read 

 of ' lakes suddenly appearing where all was dry before,' rivers and 

 lakes ' bursting- un out of the earth,' ' lightnings and clouds of smoke 



24 



