2G TRANSACTIONS OF THE [OCT. 18, 



considerable movement beneath the ocean in the vicinity would 

 have resulted in a great wave upon the shore, such as have at- 

 tended many other earthquakes, viz., that of Kingston, Jamai- 

 ca, in 1692, of Lisbon in 1755, or that of Arica, Peru, in 1868, 

 No wave at all is reported on the coast of South Carolina, 

 though a distinct shock was felt on board a ship which had just 

 left Charleston. This proves that the seat of the disturbance 

 was not under the sea, but on the land, the vibrations passing 

 from the land seaward. It is reported that a slight change in 

 the depth of the water in Charleston Harbor has resulted from 

 the earthquake, but no accurate observations have been made 

 to test the truth of the report. In all probability the movement 

 was in the old crystalline rocks beneath the comparatively mod- 

 ern deposits which underlie the surface, and consisted in a 

 yielding to lateral thrust which ruptured and slipped some of 

 the beds over others. Such fracture and movement would pro- 

 duce vibrations which would take the form of successive waves 

 passing vertically, upward and outward in every direction from the 

 focus of action. As the older rocks which underlie this region 

 dip toward the east and have a strike north and south, an im- 

 pulse produced by disruption would move north and south in 

 continuous strata, and east and west through a succession of 

 beds which would be less good conductors. Hence the area of 

 vibration will doubtless be found to be an ellipse with its longest 

 diameter north and south. 



One fact reported from Charleston is very interesting if true, 

 and that is that the railroads leading westward were shortened; 

 the rails being arched much out of line. To straighten the 

 roads, it is said to have been necessary to cut out '' here one foot,'* 

 there two feet/' etc., of the rails. This shortening, if verified 

 and measured, would give a clue to the location and extent of 

 the subterranean movements which produced the vibrations. 



The determination of the depth of the focus of action in 

 earthquakes is generally a difficult problem from the lack of ac- 

 curate observations. Where the angle of emergence of the 

 earthquake waves can be ascertained, even along a few lines, it 

 is easy to fix the point where these lines intersect. In this way 

 the depth of focus of many earthquakes has been determined. 

 Mr. Mallet, who has made the most elaborate study of earth- 

 quake phenomena, gives the maximum depth at 8^ geographical 

 miles and the minimum depth at 2f miles; but Dr. Oldham, 

 late director of the Geological Survey of India, estimated the 

 focus of action in the great Cachar (Bengal) earthquake of 1869 

 to be from twenty-five to thirty miles in depth. 



The bursting out of temporary springs or fountains of water 

 carrying quantities of sand has been mentioned as a remarkable 



