INVEETEBEATE ANIMALS OF VINEYAED SOUND, ETC. 301 



axes nearly parallel with tlie central axis of the channel, or rather parallel 

 with the direction of the tidal currents. One of these areas, south of 

 Osterville, Massachusetts, is 15 fathoms deep, but of no great size. 

 These deeper depressions are surrounded by banks and ridges of sand, 

 some of which rise nearly to the surface and form dangerous shoals -, the 

 shoals, like the deep channels, have their longer axes parallel with the 

 prevailing tidal currents, but as they are mostly composed of loose 

 moving sands, they are liable to be altered in form and position by 

 severe storms. 



These moving sands are generally very barren of life, and form true 

 submarine deserts. Included within and nearly inclosed by the 10- 

 fathom line, there is, between Martha's Vineyard and Xaushon Island, a 

 large area of shallower water, which is connected with the shallow 

 water of the shore at the northern end of Martha's Vineyard, off the 

 " West Chop," near Holmes's Hole. In some places this shallow rises 

 nearly to the surface and forms the " middle ground,"' and other shoals 

 parallel with the current that sets through the channels on either side, 

 and consequently nearly parallel with the shore of Martha's Vineyard. 

 It is evident that this rather extensive bank is due to the action of the 

 tidal current which sweeps around West Chop toward the mouth of 

 the sound, following the direction of the deeper channels, the projecting 

 point at West Chop furnishing a lee in which the movement of the 

 water is retarded and the sediment deposited; but this action is modi- 

 fied by the tidal current which enters the mouth of the sound and flows 

 in the opposite direction, for although this current is somewhat less 

 rapid, its duration is- longer, especially that branch of it which flows 

 between the Middle Ground Shoal and Martha's Vineyard, for this flows 

 eastward seven hours and twenty- six minutes, while the opposite cur- 

 rent flows w^estward for only four hours and thirty-four minutes -, the 

 effect of the current flowing eastward would, therefore, be to keep this 

 channel from filling up by the sediments carried along by the westward, 

 currents. The same effect would be produced in the main channel, out- 

 side of this shoal, although the difference in the duration of the flow 

 in the two directions is there less, the eastward flow lasting six hours 

 and fifteen minutes, while the westward tide lasts five hours and forty- 

 five minutes. 



Similar causes determine, without doubt, the position of all the other 

 shoals and banks of sand in this region, as well as the existence of the 

 isolated deep areas between them, but in many cases the direction of 

 the wind-waves produced by the more violent storms must betaken into 

 account. The 11-fathom line extends into the mouth of the sound, as 

 far as a point opposite Nashawena Island ; and beyond this there are 

 several isolated areas which are of this depth ; the most extensive of 

 these is opposite the southern half of IS'aushon Island and in a line with 

 the main channel at the mouth of the sound. Since the tides are greater 

 in Buzzard's Bay than in Vineyard Sound, and neither the times of low 



