340 Mr. T. M. Reade on Tidal Action as 



running eastward along both sides of the Gulf, which contri- 

 bute to increase the agitation of the waters*. 



The width of the channel is about two thirds of a mile, and 

 its greatest depths from 80 to 105 fathoms, while in the 

 Sound of Jura and in the sea to the westward the soundings 

 are generally much less. The greatest depth occurs just in 

 the position it should do if attributable to the excavating 

 power of the current. One of the soundings of 8G fathoms on 

 my chart shows a rock bottom, so probably the tide sweeps 

 the bottom of the channel as deeply as the rock will allow. 



To the west of Scarba, and turning at an angle from the 

 Coirebhreacain channel in the direction of Loch Linnhe, is a 

 depression or "deep," with soundings of from 113 to 124 

 fathoms. It is very probable that the action of the tide in 

 these complicated channels between the numerous islands pro- 

 duces under-currents, having a direction diverse from and 

 perhaps sometimes opposite to the surface-streams. 



To the eastward of Barra Island, from south of Barra 

 Head to Barra Sound, a distance of over 30 miles and parallel 

 to the land, is a trench from 100 to 132 fathoms deep; while 

 outside the general depth is about 80 fathoms with a muddy 

 bottom. In this trench four of the soundings show a rocky 

 bottom at depths of 104, 125, 128, and 132 fathoms ; while 

 one of 131 fathoms is marked as stony, the remainder being 

 sand, shells, or mud. Here, again, it is evident that the tidal 

 scour has eroded this channel at a depth of 780 feet. 



In the Little Minch there is a " deep " between the Isle of 

 Skyeand North Uist with a sounding of 107 fathoms, u rock," 

 the surrounding bottom, with soundings from 50 to 90 fathoms, 

 being shells, sand, or mud. 



Between the island of Bum and the island of Barra there 

 are a good many soundings showing rock, which are deeper 

 than others in the vicinity which have muddy bottoms. 



Off the island of Rathlin, north of county Antrim, there is 

 a deep or pool with soundings of 100 to 133 fathoms ; several 

 of the deepest show rock. The surrounding bottom is of 

 shells, sand, and mud at less depths. 



It seems to me pretty evident that, as in the case of the 

 " remarkable ditch," mentioned by Captain Beechy, off the 

 coast of Wigtonsbire, 20 miles long by about a mile only in 

 width, and from 400 to GOO feet deeper than the general level 

 of the bottom about it, these "deeps" off the Scotch and Irish 

 coasts have also been "scooped out" by the tidef. 



* ' Sailing Directions, West Coast of Scotland,' p. 43. 



t See "Tidal Action as a Geological Cause," l'roc. Geol, Soc. of Liver- 

 pool, 1873-4. 



