[301] INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 7 
axes nearly parallel with the central axis of the channel, orrather 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 Naushon 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 whiclr 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 westward for only four ours 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. 
Similargcauses 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 iu many cases the direction of 
the wind-waves produced by the more violent storms must betaken into 
account. The 14-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 Naushon 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 
