260 SEE— FURTHER RESEARCHES ON [April 24, 



quakes are extremely small in comparison with the depth of the 

 sea, it would perhaps be very difficult to detect resulting changes 

 of the sea bottom, except in cases whefe sinking takes place, and 

 the drop is large. In some cases of actual measurement in the 

 laying of cables the sinking has been found to be hundreds of 

 fathoms, which would be very easily recognized if the exact place 

 of former soundings could be found. But as the changes of level 

 in the sea bottom are fully as capricious as on land, we see that 

 regions where mountain formation is in progress would present 

 extreme complexity ; and unless the place were very accurately 

 known, one could not be sure that two soundings were over the 

 same spot. This difficulty would be less near known islands than 

 in the open sea, but it would be considerable in all places where 

 the ship is at the mercy of the winds and currents. 



Under the circumstances it is clear that great natural difficulty 

 would arise in the exact Hydrographic survey of the deep sea, and 

 an economic difficulty would be added, on the ground that such 

 surveys are not required in practical navigation. Yet the laying 

 and repair of cables would necessitate fairly accurate knowledge of 

 the depths, and we may hope, in spite of the growth of the wire- 

 less telegraph, that our ocean surveys are still in the infancy of 

 what they will be in another half century. 



Where trenches are being dug out by earthquakes there will 

 be the double incentive to ascertain the stage of the process and 

 the rapidity and location of the changes. These considerations may 

 contribute to our knowledge of particular regions; and, after all, 

 the changes in the larger regions of the ocean bottom are small. 



When the regions in which trenches are forming are once 

 clearly recognized, attention will naturally be centered upon them, 

 to the neglect of less disturbed areas. The most interesting re- 

 gions, from a seismological point of view, are those in which 

 islands are being uplifted and the sea bottom sinking, as near the 

 Aleutian, Kurile and Japanese islands, the Antandes, and along the 

 west coast of South America. But it may also be hoped that 

 the changes in depth near individual islands, such as Guam and 

 Martinique, will not be overlooked. Here the subsidence of the 

 bottom often takes the form of a hole rather than of a trench. Yet 



