216 DYNAMICAL GEOLOGY. 



ing by shoals or islands there is an increase of the depth and velocity, and 

 consequently an increased denuding force. South of Norwalk (long. 73° 23' 

 W.), where the breadth is reduced one half by a projecting point of Long 

 Island, the depth at center is increased from 15 to 32 fathoms. Again, south 

 of Stratford (long. 73° 6'), there is a shoal, and consequently a deepening to 

 27-|- fathoms. Again, to the eastward, south of the mouth of the Connecticut 

 River, where the Sound is narrowing toward its obstructed entrance, the 

 depth increases in 5 miles from 12-15 fathoms to 25-29 fathoms ; and then, 

 in 40 miles, nearing the entrance, to 45,50, and 55 fathoms. The accessions 

 of waters from the rivers give some aid in this deepening. Once outside, the 

 depth of the waters diminishes ; but the channel made by the scour may be 

 traced until Block Island is passed ; and the loops just south, of 30, 40, 50 

 fathoms in the bathymetric lines, suggest that it may extend in a wider form 

 nearly to the 100-fathom line. However this may be, the sea-bottom channel 

 indicated on the map southeastward of New York Bay, while rightly con- 

 sidered the former course of the Hudson River channel during a period of 

 sea-border emergence (D., 1857), probably owes its present depth out to the 

 40-fathom line, to the combined effects of drifted sands and the scouring 

 action of the ebbing waters (D., 1890). 



In the discharge of a river into a salt-water bay, the fresh Avaters flow 

 over the salt ; and in some cases so little commingling takes place that 

 shallow streams, carrying little detritus, leave uninjured the marine life of 

 the bottom. 



3. Deposition usually takes place inside of bays or estuaries wherever there 

 is an eddying of the Avaters or diminished velocity, as well as over tidal flats. 

 There is deposition also at the entrance of the bay, when the tidal waters 

 meet the sea outside, and spread and rapidly lose velocity : and at the ebb, 

 this area of deposition may become prolonged into and up the ba3^ But 

 part of the inside deposits are scoured aAvay with the next outflow. 



Deposition off shore of the detritus made by the grinding of beach sands 

 is only, to a very small degree, a result of tidal action. It is chiefly wave 

 and current Avork. The making of rijjples over sand-flats and shallow sea- 

 bottoms is partly a result of the gentle tidal inflow or outfloAv; but it is also 

 the Avork of wave-and-current movements. 



The height of the tide fixes an upper limit to tidal flats and sand-bars in 

 estuaries and bays by the limit it gives to deposition. But the seashore flats 

 along some rocky shores are a result simply of the shearing action of the 

 passing waves. 



2. Wind-made Waves and Currents. 



1. Their power. — The waves that come in from the ocean and break heavily 

 on the beaches and against the cliffs, are wind-made waves ; and those of 

 great force are made and propelled by storm-winds. Their progress is land- 

 ward ; and the break at summit takes place when the depth of water below 

 the trough equals about one half the height of the wave. The wave ad- 



