.554 On the Local and Relative Geology of Singapore ■, [June, 



oanic forces sufficing to elevate the rocks and produce eruptions at 

 different places along the lines of fissure, the system of semi-volcanic 

 hills extending from the termination of the Malayan plutonic chain to 

 Banca would he produced. Whether we admit the notion of a transla- 

 tion of waves or suppose that under the region a general volcanic pres- 

 sure was in operation, producing an expansive tendency whose super- 

 ficial manifestations varied according to the mineral structure and com- 

 position of the rocky crust and particular local intensity of force, the 

 same results would follow under the assigned conditions. 



Having in the above paper had occasion to bring together several 

 scattered notices of recent volcanic action in Sumatra and the west 

 coast of the Peninsula, it may be remarked that some general facts ap- 

 pear which it may be useful to separate from the local matters with 

 which they are mixed up. 



1 . The advance of a great wave upon the land, is a circumstance 

 common to most earthquakes on sea coasts. Mr. Darwin considers it 

 to be caused by a line of fracture being formed beneath the sea. If 

 there is a consequent sinking of the sea bed along the line, the rush of 

 the waters on both sides to restore the level would occasion first the 

 retirement of the sea from the shore and then the production of a wave 

 rolling in upon the shore. But might it not also be caused without 

 any sinking or even rending of the sea bed ? A strong blow beneath 

 the earth's crust imparting a momentary centrifugal tendency would 

 cause the sea above the point or line of impact to rise violently to a 

 height proportioned to the force of the concussion. But this wave 

 would necessarily be partly above and partly below the general level, or 

 have a hollow on each side towards which the neighbouring waters 

 would rush, and thus the same effect be produced along the adjacent 

 coast as in the former case. Mr. Darwin also mentions that places 

 situated on shallow bays suffer great damage from these waves, while 

 those seated close to the edge of profoundly deep water escape. In 

 the same manner the waves of the Indian ocean, on reaching the shal- 

 low coast of Sumatra, rise as they advance until they acquire a great 

 height. This is probably attributable to the friction of the bottom retard- 

 ing the waves while a constant succession press on from the sea behind. 

 When bays are narrow the wave will have a greater tendency to rise 



