434 Dana — Submarine Hudson River Channel. 



5-fathom area — which is an extended area on the outer side of 

 the sand-bars. 



Following the trough outward : from the 20-fathom line to 

 the 30-fathom line, the distance 7\ miles, the mean pitch is 8 

 feet a mile ; and to the 85-fathom line, about 11 miles, it is 8 

 feet a mile. At the 20-fathom line, the trough is 6 fathoms in 

 depth ; at the 35-fathom, it has its maximum depth, 16 to 20 

 fathoms, that is, 16 to 20 below the level of the bottom outside 

 of it. 



At the 35-fathom level the mean pitch outward becomes 

 slight. Through the 24 miles to the 40-fathom level it is only 

 15 inches a mile, and there the depth of trough is 13 to 16 

 fathoms. Then, from the 40-fathom level in the trough the 

 bottom is essentially level for the next 50 miles, three entries 

 of a depth of 41 fathoms being at the end of the 50 miles on 

 the map. A depth of 41 fathoms exists between the first 40 

 and 42 fathoms registered on the map ; and if put in half way 

 between, then the trough is absolutely level for 47-J miles, ex- 

 cepting oscillations within a range of 18 feet in the course of 

 it. So long a level trough is hardly to be found over a conti- 

 nent except in the bed of some tidal stream. 



According to the above, the trough has its greatest depth, 

 80 to 120 feet, where its bottom is 210 to 220 feet below the 

 surface ; and this is at the bathymetric line of 15 fathoms, so 

 that the deepest part is abreast of the bank having a 15 fathom 

 limit. This depth continues with little diminution until the 

 bathymetric line of 20 fathoms is reached, just beyond which 

 the channel begins its 47-| mile level of 41 fathoms. Abreast 

 of this level it loses gradually its depth by the slope of the 

 general bottom outside of it, until the 40-fathom bathymetric 

 line is reached, where the channel disappears. 



Effects of scour. — If the work is that of tidal action the 

 effects of scour decline greatly at the depth in the channel of 

 210 feet (35 fathoms), and cease entirely at that of 246 (41 

 fathoms, on the bathymetric line of 20 or 21 fathoms, only 35 

 miles off the Barnegat beach. 



It may be urged that a river channel might vary in like man- 

 ner; and this cannot be denied. Prof. Bache's map of cotidal 

 lines of 1854 and 1857, shows that the next incoming tidal waves 

 would have reached the outflowing current not far from, if not 

 inside of, the 20-fathom line. And it seems probable that 

 either this has occasioned a cessation of farther deepening by 

 scour, or else the depth alone, as a consequence of passing the 

 limit where the ebbing waters have abrading action. Obser- 

 vations on the currents may give a positive decision of the 

 question. 



The breadth of the channel, according to the map, is mostly 

 1 to 2 miles ; so that a cross section of the deeper part would 



