6o Papers from the Marine Biological Laboratory at Tortugas. 



wind-formed currents and the counter current are concordant and they 

 therefore cooperate. The tidal impulse, however, is roughly from north 

 to south and the direction of tidal currents is therefore transverse to the 

 other currents. 



The principal arcs of the rims of both the Marquesas and the Tortugas, 

 i. e., those from the southeast to the northwest entrance, are bowed toward 

 the east, against the prevailing direction of the wind and that of the counter 

 current, while the northern limb of the arc in each instance trails with 

 these currents. These arcs therefore satisfy the requirements of wave and 

 current-shaped arcs. 



The southern sides of both the Marquesas and Tortugas are broken by 

 southeast and southwest passages. These breaks are probably to be 

 attributed to cross-tidal currents. Also both the Marquesas and Tortugas 

 have a northwest passage. The explanation of this is probably as follows: 

 In the Tortugas ; although the data on currents are inexact, the available 

 evidence indicates a stronger tidal inflow than outflow, much of the water 

 of the receding tides not passing through the lagoon but around the outside 

 of its rim. The water coming into the lagoon would tend to be driven 

 toward the west by the winds, and would consequently find its outlet in 

 the northwest quadrant. 



In the Marquesas, the land area west of the southeast passage follows a 

 line that would be expected of the southeast horn of a crescent formed 

 under conditions of wind and currents known to prevail there. The 

 formation of the bank and the small keys on it on the west side of the 

 lagoon is more obscure. Perhaps they were formed by winds and accom- 

 panying waves that come, especially during hurricanes, from the west and 

 by a current eddy produced to the leeward of the bow of the crescent. 



The main agencies whereby the principal arc of the rim of the Tortugas 

 was formed seem definitely known, but those building other parts of the 

 rim are still somewhat vague. Between what is known as Five-foot Channel 

 and Southwest Passage are holes, some of which are 6 fathoms or more in 

 depth. It is evident from the distribution of these holes that once there 

 were several channels trending more or less from southwest to northeast 

 and approximately following the course of tidal flow, the channels bounded 

 by banks elongate in the same direction. Detritus carried westward by 

 waves and currents has filled parts of the channels and broken them up 

 into holes, some series of which may still be recognized. 



Loggerhead Key, as well as the bank on which it stands, is elongate 

 from southwest to northeast and has each of its ends somewhat curved 

 toward the west. The general direction of the bank is approximately along 

 that of tidal flow, but it is not an unmiodified linear ridge coinciding in 

 direction with that of one current, for it is evident that the prevailing 

 run of the waves before the wind, and perhaps the counter current acting 

 at its southern end, have curved its ends westward. The effect of waves 

 running before the wind in shaping a key was studied in detail on Logger- 



