SUBSTRATA 1D 
the mud covering the bottom of the harbor. The upper, 7. e., inshore, edge of 
the peat belt becomes fairly well drained at low water, due largely to the number 
of burrows of the fiddler crab Gelasumus pugilator. Toward the middle and 
lower edge of the Spartina belt the soil is less firm and is poorly drained, except 
close to the edge of stream-channels and, in some places, along its own abrupt 
lower border, at the 1.5-foot level. In these places only do the fiddler-crab 
burrows, and an occasional muskrat burrow, afford some drainage and an 
opportunity for aeration. 
The only plants usually occurring on this peaty soil besides Spartina glabra 
are the alge Rhizoclonium, Enteromorpha clathrata, Fucus vesiculosus spiralis, 
and occasionally Lyngbyas, which are matted about the Spartina stalks or over 
the mud. Near the upper margin of this zone of soil, however, occasional 
inwandering seed plants from the higher levels may be encountered. Of these 
the most often found are Solidago sempervirens and Sueda. | 
On the gravelly soils of the stream-beds there occur Lileopsis at 3 to 5 feet 
Triglochin and Plantago decipiens near 6 feet, and the alge Hnteromorpha 
intestinalis, Monostroma, Ilea, and Hildenbrandia. 
The soil of the zone above the 6.5-foot level differs much more at different 
parts of the boundary of the harbor than that below this level, the character at 
each point depending apparently on the supply of fresh water and on the plant- 
covering of the same part of the shore just above high-tide level. 
On the Spit, e. g., the soils above 8 feet are sandy and dry. In correlation 
with this we often find between 6.5 and 8 feet a sandy or gravelly soil, with little 
humus, occupied by felts of alge or by Salicornia and Sueda. Trese gravelly 
and sandy stretches are perhaps due primarily to wave-action, for when the 
dead Spartina stalks have been broken off in the fall, the waves raised by the 
strong winter winds beat with considerable force against this south shore of 
the Spit. Alternating with these areas of gravelly soil stretches containing 
more humus are found, which are occupied chiefly by Spartina patens and 
INstichlis. 
On the shaded west shore, and on the east shore south of the mill, we find 
these levels, except for the narrow stream-beds, furnished with a damp, humus- 
containing soil that is sometimes very peat-like in character. This usually 
grades off insensibly below into the peaty substratum of the Spartina glabra. 
Above this there is often a sharp cliff-like drop at the boundary between the 
soil of this belt and the soil of the supra-littoral belt, near the 7.5-foot or 
8-foot level. At the bottom of this little escarpment, of a decimeter or two in 
height, the gravelly or sandy subsoil is often nearly bare of mud. The vegeta- 
tion of this soil of the upper littoral belt on the east and west shores consists in 
the better-drained areas, chiefly of Spartina patens, and where fresh water is 
present chiefly of Scirpus americanus. In with these are scattered Scirpus 
robustus (near fresh water), Solidago sempervirens, and, on more sandy areas, 
Spergularia (plates x1r and x111). Along the rivulets and streams plants of 
the supra-littoral belt may push down below the 8-foot level. 
Along the southern boundary of the harbor, at the north edge of the Marsh, 
the most marked change in the character of the soil in passing upward from the 
6.5-foot level is in the greater firmness and probably larger percentage of organic 
content of the soil, as it rises abruptly from the 6-foot to the 7-foot level and 
then slopes up very gradually to the 8-foot or 9-foot level. The upper layer of 
