102 ON ROCK REMAINS IN THE BED OF THE ENGLISH CHANNEL. 



the limited capacity of the dredge in regard to those positions where 

 the stones are exceptionally numerous. But as a general estimate I do 

 not think it is very far short of the truth, having reference, of course, 

 only to stones exposed at the surface or very little covered. The most 

 reliable test is probably to be found in the result of hauls with the 

 conical dredge and the 1' 6" dredge. The former of these instruments 

 is so constituted as to dig deeply with its heavy frame into the sea- 

 bottom, and is therefore allowed to work only for a fraction of a 

 minute, during which time the canvas bag with which it is fitted is 

 rapidly filled with a complete sample of the bottom from a very small 

 area, about 4 to 8 square yards. The 1' 6 " dredge was also fitted with 

 a canvas bag, and though not digging so deeply was used for a very 

 similar purpose. Of five hauls with the conical dredge, from 19 to 49 

 miles, two produced no stones at all ; of six hauls with the 1' 6" dredge, 

 from 30 to 46 miles, three produced no stones at all; but if these 11 

 hauls be summarised, and the number of stones of 4 cm. and over be 

 distributed over the sum of the estimated areas covered, a ratio of 

 distribution is obtained for these short hauls very nearly equiva- 

 lent to that for the whole area of the work, namely, 0"6 per square 

 yard. 



With reference to this absence of stones in five hauls with these small 

 dredges, there is no doubt that the stones lie exposed on the sea- 

 bottom very much more thickly in some places than in others, owing 

 to the varying degree of sedimentary deposit in different areas depen- 

 dent on tidal action and the physical conditions influencing animal 

 settlement, and so regulating the local deposition of shell and other 

 debris of animal origin. In the " Distribution " column of the table 

 it will be seen that the ratio varies as widely as from 0"4 to 28-0 

 square yards per stone. An interesting case in point occurs in tlie 

 Positions (10) to (13), S. 26' W., 18 miles. These four samples were 

 taken in quick succession in a westerly direction over a distance of 

 about 1 mile, and lying thus close together, are shown on the chart as 

 coincident. It seems evident that here the dredgings passed through 

 the middle of a stony patch which was almost covered by finer 

 deposits at either limit. 



At (10) 6 stones were obtained with an average distribution 



of 1 stone to 28 sq. yds. 

 „ (11) 166 „ „ „ „ 2 „ 



„ (12) 187 „ „ „ „ 1-8 „ 



„ (13) 21 „ „ „ „ 27-3 „ 



To quote another instance, at Position (17), S. 28° W., 23-3 miles, 

 a haul of 11 minutes with the 3' 0" dredge produced only 15 stones, 



