OF THE BASIN OF MINAS. 2399 
only in localities sheltered from the action of too strong 
currents. In these accumulations there is thrown down 
a layer for each tide, those deposited by the night tides 
being thicker than those formed during the day. During 
a long interval of repose thin layers only are deposited, 
while just after a heavy storm that stirs up the whole bay, 
the deposits are much increased in thickness. Nor are 
these accumulations confined to the shoals laid bare at 
low tide; but they extend over the bottom of the Basin, 
though they must naturally be much thicker near the 
shores whence the materials were originally derived. Dr. 
Dawson says that “these layers are thicker on the edge of 
the flats than near the shore; and hence these flats, as 
well as the marshes, are usually higher near the channels, 
than at the inner edge. From the same cause, the more 
rapid deposition of the coarse sediment, the lower side of 
the layer is arenaceous, and sometimes dotted with films 
of mica, while the upper side is fine and shining, and 
when dry has a shining and polished surface. The falling 
tide has little effect on their deposits, and hence the 
growth of these flats, until they reach such a height that 
they can be overflowed only by the highest tides.” 
It is to the zone embraced between high and low-water 
marks that the subaerial tracks, such as we have described, 
are confined, the only markings made on the submerged 
layers being entirely those of aquatic animals, tracks of 
crabs and other crustaceans, trails of shell-fish and marine 
worms, or scratches made by fishes; but markings of this 
kind may extend over the whole part reached by the tide, 
while low tide is the lowest limit at which tracks of land 
animals can be found. 
The Tringw and other waders visit the shores of the 
Basin only in the summer, so that through the whole 
