376 ox THK FAUNA AND BOTTOM-DEPOSITS NEAR THE 30-FM. LINE 



The evidence for considerable wave action is strongest on Grounds 

 XIA''. and XVI., immediately to the west of the Eddystone reef, where 

 it is probably increased by the rebound of the Atlantic swells from the 

 rocks. On Ground XVI. (see Charts) the bottom-deposit is a shell 

 gravel, which is remarkably clean and free from silt,* as will be seen 

 from the figures in Table II., p. 525, which shows the texture of the 

 samples taken from the different grounds. There is also on this ground 

 au almost entire absence of such species as fix directly to the bottom- 

 deposit. 



The sample of the bottom-deposit on Ground XIV. (haul 85) is 

 unique amongst all the samples obtained, in having the fine sand grains 

 rounded and highly polished (see p. 385), and on this ground the 

 number of hydroids is also very small. 



There is evidence (see p. 425) that Ground XIII., which lies south- 

 east of the reef, is more sheltered from wave disturbance than the 

 grounds to the westward of the Eddystone. 



2. Currents and Tides. Practically nothing is at present known as to 

 the strength or even as to the prevailing direction of the bottom 

 currents of the Channel. It is well known, however, that the surface 

 currents are especially strong around the various headlands, and it 

 is probable that these will still be felt on the bottom. If this be so the 

 currents on the Bolt Head and Prawle Grounds may be more rapid than 

 those in the neighbourhood of the Eddystone. 



Temperature. The Laboratory records of bottom temperatures at 

 depths of about 30 fathoms show a range from 7"1° C. (48° F., 

 March, 1898) to 13-9° C. (571° F., Nov. 17th, 1898). The records at 

 this depth are not, however, very numerous, and the above observa- 

 tions probably do not quite represent the maximum and minimum 

 temperatures. The temperature changes very slowly, generally not 

 more than 1° C. in a month, and the total yearly range may be 

 safely put at about 7° or 8° C. 



Pressure. The depth of water on all the grounds varies only from 

 25 to 35 fathoms, which represents a difference of pressure too slight to 

 affect the bottom animals. 



Amount of liyht ivhich penetrates. This is sufficient only for the 

 lowest forms of vegetable life, such as diatoms, coccoliths, etc. Xone 

 of the branched red or brown sea-weeds are found on any of the 

 grounds. 



* Wave action in itself would not of course be able to remove silt which had been 

 deposited ; it would, however, tend to keep the silt in suspension, so that it would be 

 carried away liy llio prevailing currents. 



