ON THE FACTS OF EARTHQUAKE ΡΗΦΝΟΜΕΝΑ., 39 
There is no voleanic rock of any sort, he affirms, in any part of Calabria 
visited by the earthquake of 1783. 
The great plain consists of a vast collection, as it were, of small table- 
lands, separated by deep ravines, having steep escarpments cut into the clay 
and sand or sandstone by the action of the rivers and torrents. These ravines 
are often 500 to 600 feet in depth below the table-land, which is highly cul- 
.tivated above them, and the sides of these ravines are of the dense clay or 
scarcely coherent sandstone. All these slope up and abut against the sides 
of the Apennines, which form the axial line of the country. 
This somewhat tedious account is necessary to make the remarks here- 
after to be made as to the secondary effects of earthquakes intelligible. 
The centre of effort in this earthquake was under the great plain, and pro- 
bably about under where once stood the village of Oppido, but at an unknown 
depth. 
The observations made amount to no more than this; that the shocks did 
less mischief to structures on the granite or slate rocks of the hills than they 
did to those on the plain of clay, &c.; that the destructive effects of the 
shocks were very great along the line of junction of these, at the bases of the 
hills (from which some of the philosophers of that time concluded that the 
earthquake came from the mountains), and that along this line, shocks in 
close succession were felt, not only horizontally and vertically, but also in 
opposite directions. 
Now we may ἃ priori account for these facts, on the principle that the 
velocity of the shock or earth-wave depending on the density and modulus of 
elasticity of the formation through which it passes, and its velocity being 
greatest in those whose elasticity is highest, while its range of motion ‘is most 
limited in the same ; therefore the shock here was of less velocity in the plain 
than in the rocky hills; but had in the former a longer range of oscillation, 
and hence did most mischief in the plain. Along the line or plane of junc- 
tion of two formations of different elasticities, &c., the earth-wave will change 
_ its course and also its velocity (like light in passing from one medium to 
another); and here the wave will be divided, part of it will be refracted, and 
part will be reflected (or total reflexion may take place if the angle of inci- 
' dence be suitable at the plane of junction); and the latter portion of the 
wave will in such case double back upon itself, and give rise to a shock in 
the opposite direction to the first one. Hence along such a line of junction 
the destructive effects will be very great. Although the direction of transit 
of part of the shock is changed by thus passing from one formation to, an- 
other, and its force also modified, yet it often happens that such changes do 
μοῦ arrest much of its main progress or effects. 
_ Even large ranges of mountains abutting on plains of soft material are 
shaken through and through, and the shock is transferred beyond them, Thus 
the shock at Lahore in India of 1832, passed through the chain of Hindu- 
_ Cutch to the Upper Oxus, and even to Bokhara; and in South America they 
pass through “the littoral chains of Venezuela and the Sierra Parime.” 
(Humboldt, ‘ Cosmos.’) 
_ Such differences in effects of shock due to situation have been repeatedly 
| 
observed. Humboldt says that at Quito, which stands at the foot of the 
active volcano of Rura Pichincha, 8950 feet above the sea, and contains large 
and lofty buildings, with spires and domes, he has been often surprised at 
the severity of the shocks which he has felt, and which nevertheless but 
ΤΡΆΓΟΙ. rent the walls; whilst in the plains of Peru much weaker oscillations 
injured even lowly houses built of cane ; and many other instances might be 
quoted. To myself the explanation of the facts which theory gives appears, 
