496 THE JOURNAL OF GEOLOGY. 



tension existing among the fine particles of a solid in suspension, 

 which are modified by the presence of salts. 1 But whatever 

 the conclusion may be as to the nature of the controlling law, 

 the influence of salt water in this respect is an important cause 

 of deposition of clays at the mouths of rivers. 



(£■) Inequalities of depths ; lee banks. — When any volume of 

 flowing water expands, it loses velocity and, if muddy, deposits 

 sediment. This well recognized condition of river deposition 

 has been considered in reference to a river entering a lake ; it is 

 equally true of an ocean current or of undertow, where the for- 

 mer passes from a narrow strait to the broader sea, or where 

 either one flows from shallow into rapidly deepening water. The 

 condition needs no explanation — it requires only illustration. 



From the Atlantic the southern equatorial current sweeps past 

 the mouth of the Amazon and Orinoco ; as the Gulf stream it 

 crosses before the Mississippi delta, and pouring out through the 

 Straits of Florida enters the North Atlantic. From the rivers tribu- 

 tary to its course it receives fine sediment escaped beyond the deltas. 

 In its passage through the Caribbean sea and the Gulf of Mexico 

 it flows over the eastern Caribbean deep, Bartlett's deep and 

 Sigsbee's deep, and where it leaves the Blake plateau north of 

 the Bahamas it falls over the continental rim into ocean depths. 

 Between these basins it traverses relatively shallow seas, whose 

 bottoms are floored with modern limestone and green sand. 

 These deeps of 2,500 to 3,000 fathoms and shoals at 100 to 500 

 fathoms are result of epeirogenic forces probably, but they are 

 now floored with deposits which consist of the shells of pelagic 

 organisms mingled with terrigenous silt, forming " modified 

 pteropod ooze." 2 This deposition, if it has gone on long enough 

 since the depression at the deeps, or fast enough to mask the 

 details of deformation, possibly continued up to a recent time, 

 determines the profiles of the slopes from shoal to abyss. In 



1 U. S. Dept. Agric. Weather Bull. No. 4, 1892, "Some physical properties of 

 soils," pp. 19-23. Milton Whitney. 



2 Geologic and bathymetric maps of the Atlantic in " Three Cruises of the Blake," 

 by Alex. Agassiz, Vol. I. 



