38 TEANSACTIONS OF THE [NOV. 1, 



These earthquakes usually, it might almost be said always, 

 occurred or were most severe in the deltas of great rivers or in 

 such proximity to them as to point to some association of such 

 rivers with their origin. Such were the earthquakes of Catch at 

 the mouth of the Indus, of Lisbon at the outlet of the Tagus of 

 of 1662 and 1791, in the gulf of St. Lawrence, of New Madrid 

 in 1811 in the valley of the Mississippi. Even those of Chili 

 were most destructive at river-mouths, like that of 1837 de- 

 scribed by Darwin, which destroyed ConcepQion in the valley of 

 the Biobio Kiver. It might even be possible to include in the 

 group the so-called Charleston earthquake of 1886. 



To learn what happens at the outlet of a great system of river 

 drainage, we could have no better example than the Mississippi. 

 With its branches it is the longest, its tributaries are the most 

 numerous and the largest, and it drains the greatest area of any 

 upon the earth. If the computations of engineers are reliable, 

 it has in the past deposited a delta which commences above the 

 Ohio River, has a superficial area of 13,600 square miles and an 

 average depth of five hundred and twenty-eight feet. Every 

 year it gathers up its burden from a vast plain, transports one 

 thousand miles southward and deposits in the regular basin, 

 called the gulf, 3,702,758,400 cubic feet of solid matter. At 

 thirteen cubic feet to the ton, the annual tonnage of this carrier 

 by water is 284,827,569 tons. 



Such a weight added year by year, principally deposited around 

 and not far from the outlet, must in time become too great to be 

 sustained by the earth crust beneath and the crust must give 

 way. When it does, there will be a depression into the fluid 

 below, which must be compensated by an elevation elsewhere. 



If the subsidence is gradual, there may be no surface disturb- 

 ance. But the crust is not homogeneous, is more rigid and less 

 flexible in some places than in others. Suppose it resists the 

 pressure in some places and yields to it in others for a time, and 

 then at the weakest point suddenly gives way. That point may 

 be anywhere within a circle large enough to include Charleston. 

 Wherever it is, there will be a sudden disturbance of levels, not 

 only on the surface, but in the interior contents. It will give 

 birth to an earthquake wave which, once set in motion, will, like 



