OF SUMATRA. 1 3 



gicferable, and promote, in no small degree, the ele- 

 vation of islands. Earthquakes are very frequently 

 felt on this island, and on the contiguous ones. Se- 

 veral shocks are sometimes experienced during the 

 course of a month. It is observable that this tremend 

 phenomenon, in its progress, undulates the space ft 

 moves, or travels, under; and that the concave p 

 of these undulations open into fissures when the 

 motion is violent. It is not improbable but that 

 such openings take place under shoals, 01 immediate- 

 ly contiguous to them. In this case, to preserve the 

 equilibrium, it seems reasonable to suppose that the 

 surrounding sand and substances will rush in, hurried 

 along by the general movement, in a greater quan- 

 tity, from the degree of momentum impelling them, 

 than what occupied the space of the fissure when at 

 rest. These hiatus take place '-only on the side of 

 the undulation, from which the earthquake proceeds; 

 and the sand on that side, now inclining to rest, after 

 having experienced the shock, but still possessing a 

 tendency to move in the direction of the earthquake, 

 will naturally fall into the hiatus opened for its re- 

 ception, before the undulation can reverberate into 

 its original position. Hence the shoal, or island, 

 will be in some degree raised, by an effect similar to 

 that of a lever, though by different means. These 

 islands and shoals, being further removed than other 

 parts exposed to the shock, from the subterraneous 

 or submarine crannies or channels in which the earth- 

 quake acts, will, of course, resist its action more 

 than- parts possessing less incumbent weight. The 



undulations 



