74 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Juiie 13, 



waves, inasmucli as in calm weather, wlien I have had opportunities 

 of passing over some of these platforms, I have felt convinced that 

 the only other agent, namely the tidal streams, had not sufficient 

 velocity to exercise any mechanical power whatever. 



With the present tendencies of geological speculations, it is of 

 some interest to ascertain, if possible, the depths at which the sea- 

 bed is liable to disturbance : on the determination depends the know- 

 ledge we may some day acquire of the conditions of accumulation, 

 which the older stratified deposits exhibit. 



Whenever a stream of water flows over a rocky or uneven bed, the 

 interruption which the lower stratum experiences is indicated by a 

 v>^ave-disturbance at the surface : of this every running stream is a 

 ready illustration. In parts of our seas, surfaces of broken water, 

 known as races and overfalls, are constantly met with, and may even be 

 observed from the land. The race of Portland occurs over a ledge 

 of rock which runs out from the south extremity of the Bill ; the 

 east and west sides of this ledge are steep : the case of the St. Alban's 

 race is precisely similar. The sm-face-cUsturbance at these places is 

 to be obsen ed in the calmest weather, and in diifting over them at 

 such times, the passage from smooth into broken water is immediate. 

 In like manner the race of Barfleur, more formidable than either of 

 the preceding, occurs over a ledge of rocks, running out from the 

 headland of that name. As in all cases, the greatest distm'bance, apart 

 from the action of the wind, takes place at new and full moon, when 

 the tide-stream flows with a velocity of from eight to nine miles an 

 hour ; showing that the races are due to the arrest of the tidal stream 

 by these ledges. The Boulogne fishemien sink their nets athwart 

 the deeps at the east end of the Channel ; should the weather become 

 too rough to allow them to get them in, they are sure to recover them 

 on the coast between Cape Gris Nez and Calais, whither the flood tide 

 diifts them in. These and many like cases to be collected along the 

 coast show that the tidal movement extends to the whole depth of 

 water. These cases are to be observed along the marginal zone, and 

 some persons have drawn a distinction between the movement of the 

 tide in deep as compared with shallow water. A broken or rippled 

 surface is met with at certain places in the open part of the Channel, as 

 along the entrance fi'om the Atlantic : these appearances are in every 

 instance connected with uneven ground below. The ripplings over 

 Jones's Bank are very considerable ; the shoalest part of this bank has 

 40 fathoms, with a surrounding sea-bed at 70. A hke surface dis- 

 tm'bance is constant along the edge of the Nymph Bank, which has 

 a minimum depth of 45 fathoms. The Little Sole Bank has like in- 

 dications even in the calmest weather : over the simmiits of this 

 group there is a depth of 60 fathoms, with 100 fathoms at short di- 

 stances around. In all these instances the place and extent of the 

 ripphngs depend on the direction and strength of the tide : the slopes 

 of all these banks are steep ; the process is the same as with the 

 shallower ledges, and shows, according to what theory requires, that 

 the tidal stream movement extends to the whole depth of the Channel. 



In the earliest accounts we have of the Channel and its dangers, we 



