Mr. Matuet on the Dynamics of Earthquakes. 75 
To apply this to our subject: when the earth wave passes abruptly from a 
formation of high elasticity to one of low elasticity, or vice versd, it will be partly 
reflected; a wave will be sent back again, producing a shock in the opposite 
direction ; it will be partly refracted, that is to say, its course onwards will be 
changed, and shocks will be felt upwards and downwards, and to the right and 
left of the original line of transit of the wave. This is exactly what has been 
observed to take place. Thus, Dolomieu informs us 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 any where these were 
situated in the mount ains.* 
Here the transit of the wave was from the clay and gravel, which have the 
lowest possible elasticity, into the granite, whose elasticity is remarkably high ; 
and hence the shock, after doing great damage by varying its direction, and 
returning upon itself, at the junction, was at once eased when it got into the 
elastic material of 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 lying in a small- 
sized valley), and pass across this into similar elastic rock at the opposite side, all 
the former results will follow ; but, in addition, the whole mass of clay, or sand 
and gravel, in the basin of the valley, will be shaken as a whole by any powerful 
shock, which will be felt over its whole area at the same instant; in other words, 
the contents of the basin or valley will be constrained to vibrate as one system, 
with its walls, namely, the elastic rock of its sides. This gives the solution of the 
fact frequently recorded of places so circumstanced, and at not very great dis- 
tances, feeling the shock of the one earthquake at the one moment. 
We have thus traced many of the variable and secondary effects of the transit 
of the great earth wave, and may remark, in concluding this branch of our 
subject, that earthquakes must be regarded neither as the cause nor as the imme- 
diate effects of the elevation of a district of the earth’s surface, but merely as the 
remote effects of elevations occurring at a remote centre, so that the true defini- 
tion of an earthquake is, the transit of a wave of elastic compression in any 
* See also M, Place’s Account of the Earthquake in Chili, 1822, Quarterly Journal, vol. xvii. 
p- 42. 
Tee 
