THE FORAMINIFERA OF THE EXE ESTUARY. 337 



to that width and depth through which the waters of the ebl) will 

 flow witli sufficient velocity to maintain a fairly constant cross-section. 



The surface of the Polesands, turned over by wave and current 

 action even in calm weather, contains little or no organic matter, and 

 any Foraminifera attempting to establish themselves thereon are liable 

 on eacli tide to be buried at a considerable depth below the surface. 



The average least diameter of the largest sand grains on the seaward 

 slopes of the Polesands is 0"7 mm., the average least diameter of the 

 largest sand grains on the estuarine slope of those sands is 04 mm. ; 

 in each case the average greatest diameter would be about 50 per cent, 

 more. The figures were obtained by ranging a number of grains 

 between parallel plates. The average least diameter of the largest 

 sand grains at low- water mark on the north shore of the Warren is 

 0-25 mm. 



To move a grain of quartz sand of 1 mm. diameter requires a current 

 of velocity 0"5 feet per second, or, say, one-third of a knot ; while to 

 move a grain of 0'2o mm. requires a velocity of 0'25 feet per second, 

 or, say, one-sixth of a knot. 



If the sand is once allowed to compact thoroughly with a reasonable 

 admixture of silt, a much higher velocity is required to move it, say, up 

 to ten times the figures above given. 



To anyone familiar with the actual tidal currents at Exmouth the 

 unstable nature of the surface of many of the sand banks, especially 

 below half-tide level, will be at once apparent. 



According to King's Channel Pilot : — 



" When the banks at the entrance (of the estuary) are covered, both flood 

 and ebb streams set fairly over them, about 2| knots ; but when uncovered, 

 these streams run strong through the channel, and their strength increases at 

 Ferry Point to 5 knots." 



The channel followed by the ebb across the Polesands at the end 

 of the Warren, until these sands are uncovered, can be clearly seen on 

 the chart of the harbour. 



It is to be noted that the grains on the inner side of the Polesands 

 are much better polished and rounded than those on the outer. The 

 former travel some little distance to and fro with each ebb and flood, 

 the latter are chiefly subject to wave-action. 



The Warren sand is richer in organic matter as it rises in level from 

 low-water toward high-water level, but it is not to be understood that 

 the increase in organic matter is directly proportioned to level. 



Near Lympstone a very fine sand occurs, the average least diameter of 

 the largest grains being only 0"12 mm. 



The conditions being so unfavourable, it is not surprising that 

 Foraminifera are scarce. 



