GEOLOGY. 



397 



rally considered more frequent near to coasts ; thus, 

 Syria, the coasts and islands of Asia, America, the 

 European coasts of the Mediterranean, and Iceland, 

 are most subject to them ; while the plains of Africa, 

 Asia, and the North of Europe are least exposed. 

 Viewing the whole earth, and including every slighter 

 agitation, earthquakes appear to be exceedingly 

 numerous, and it may be maintained that not a week 

 passes in which the earth's surface, in some place or 

 other, is not more or less agitated. The great num- 

 ber of concussions observed in civilized countries, 

 and the fact that some districts are constantly agitated 

 by them, entitles us to draw the conclusion. Their 

 return in the places most subject to them, and in the 

 places where they are less frequent, is not regulated 

 by any precise period of time. Their appearance is 

 not connected with any particular season of the year 

 or state of the atmosphere, and they take place by 

 day as well as by night. 



The phenomena peculiar to earthquakes are in 

 themselves sufficiently simple. They consist in 

 tremblings and oscillations of the earth's surface, 

 called shocks ; extending over greater or smaller 

 tracts of country, and frequently following a parti- 

 cular direction. The shocks appear at first chiefly 

 as perpendicular heavings ; then as horizontal un- 

 dulations or oscillations ; lastly, in some instances, 

 there is a violent agitation : the motion is more or 

 less rotatory. If to these we add the rending, slip- 

 ping, rising, and sinking of the ground, the violent 

 agitations of the sea, lakes, rivers, and springs ; con- 

 sisting, in springs, in their drying up or (bursting 

 forth with great violence ; in lakes, rivers, and the 

 ocean, in their falling and rising, and rushing back- 

 wards and forwards, owing to the sinking and rising 

 of the land, we obtain an enumeration of the princi- 

 pal phenomena. 



The slighter shocks of an earthquake, consisting 

 of perpendicular heavings and horizontal undulations, 

 commonly produce rents in houses, moving light 

 objects in them, as articles of furniture. Persons 

 unacquainted with the phenomenon, or who do not 

 perceive it from the subterraneous noise resembling 

 thunder which accompanies it, feel unsteady while in 

 their beds, but particularly when sitting, and believe 

 themselves seized with a sudden giddiness. The 

 shocks proceed gradually to be more violent, and 

 then they are very easily perceived even by the in- 

 experienced. Then the most substantial buildings are 

 shattered to pieces, and the inhabitants buried be- 

 neath their ruins ; while buildings of a lighter con- 

 struction are only rent, and very slender reed huts 

 are least of all exposed to destruction. In some 

 cases the fracturing, or as it were trituration, sur- 

 passes description. Hence, for the plainest reasons, 

 it is most dangerous to remain in houses or inhabited 

 places ; but even the fields and mountains themselves 

 afford no perfect security, inasmuch as the fields fre- 

 quently in some places open into fissures, and are 

 rent asunder ; while mountains are not only rent, 

 but slide down into the valleys, dam up rivers, form 

 lakes, and cause inundations. Although the deso- 

 lation produced by these convulsions exceeds all 

 description, this is much more the case with the rota- 

 tory motions ; a species of motion, however, the 

 existence of which has been denied by some geolo- 

 gists. In proof of it, however, it may be mentioned, 

 that during the earthquake of Catania, whose general 

 direction was from S. E. to N. W., many statues 

 were turned round, and a large mass of rock was 

 turned 25 from south to east. But the rotatory 

 motion was more strikingly exemplified in the earth- 

 quake at Valparaiso, on the 19th November, 1822, 

 by which many houses were turned round, and three 

 palm trees were found twisted round one another 



like willows. These rotatory motions of masses of 

 rock jire particularly interesting whtn viewed in 

 connexion with phenomena connected with faults or 

 shifts among strata in non-volcanic districts. In the 

 earthquake at Calabria, two obelisks placed at the 

 extremities of a magnificent fagade in the convent 

 of S. Bruno, in a small town called Stefano del Bosco, 

 were observed to have undergone a movement of a 

 singular kind. The shock which agitated the building 

 is described as having been horizontal and verticose. 

 The pedestal of each obelisk remained in its original 

 place; but the separate stones above were turned 

 partially round, and removed in some instances nine 

 inches from their position without falling. See Plate 

 41, figure 1, for a representation of this. 



It is the agitation of the sea that points out the 

 great extent of the tracts of land convulsed by earth- 

 quakes. In this respect, the earthquake at Lisbon, 

 in 1755. was the most remarkable and most violent 

 that ever visited Europe. In consequence of it, by 

 the concussion on the bottom, or momentary rising 

 or upheaving of the sub-marine land, the sea over- 

 flowed the coasts of Sweden, England, and Spain, 

 also the coasts of Antigua, Barbacloes, and Mar- 

 tinique hi America. In Barbadoes the tide, which 

 rises only twenty-eight inches, rose twenty feet in 

 the bay of Carlisle, and the water appeared as black 

 as ink, owing probably to bituminous matter thrown 

 up from the bed of the ocean. On the 1st of No- 

 vember, when the concussion was most violent, the 

 water at Gaudaloupe retreated twice, and on its 

 return rose in the channel of the island to the height 

 of from ten to twelve feet. Similar appearances 

 were witnessed at Martinique. A wave of the sea, 

 sixty feet high, overflowed a part of the city of Cadiz ; 

 and the lakes of Switzerland, such as Geneva, were 

 observed to be in commotion six hours after the first 

 shock. It is also remarkable that agitations were 

 noticed in lake Ontario, in October, 1755. During 

 the earthquake at Lima. 1586, a wave of the sea 

 rose eighty-four feet high in the harbour of Callao. 

 During the earthquakes in Calabria in 1783, the sea 

 not only overflowed the coast and drowned many 

 people, but was in general so much agitated that the 

 guns on ship-board sprang from the deck to a height 

 of several inches. 



Besides the common operations of earthquakes 

 already mentioned, others occur that do not imme- 

 diately succeed the concussions, and therefore happen 

 less frequently. To these belong the sliding down 

 of parts of mountains, as at Dobratch in 1345, and 

 the falling together of two mountains in Jamaica in 

 1692, by which the bed of a river was dammed up. 

 In the latter place, a part of a mountain slid down 

 and covered many plantations ; the city of Port 

 Royal sunk to the depth of eight fathoms; and a 

 plain of 1000 acres fell in, with all buildings upon it. 



The magnitude of rents formed by earthquakes 

 vary from a few feet to many fathoms in extent. 

 They have either a direction which is nearly straight 

 or more or less winding, or they run in all directions 

 from a centre. During the terrible Calabrian earth- 

 quakes of 1783, rents were formed of great dimen- 

 sions. At Jerocarne the country was lacerated in a 

 most extraordinary manner, the fissures running in 

 every direction like cracks on a broken pane of 

 glass. (See Plate 41, figure 2, for a representation 

 of this.) On the sloping side of a hill near Oppido, 

 a great chasm opened in the form of an amphitheatre 

 500 feet long and 200 deep. See Plate 4i , figure 3, 

 for a representation of this. See too figure 4, for a 

 representation of another chasm in the hill of St 

 Angelo, near Soriano. In the territory of San Fili 

 there was formed a rent half a mile long, two feet 

 and a half broad, and twenty-five feet deep ; in the 



