126 THE RELATION OF PLANTS TO TIDE-LEVELS 
soil on this gently sloping portion consists of 2 or 3 dm. of tough, black, fibrous 
peat, bound together by the dead and living rhizomes and roots of the Spartina 
patens, of Distichlis, and of Scirpus americanus, by which this part of the 
Marsh is chiefly covered. A study of the subsoil of this Marsh, by the aid of the 
section cut out by the Creek, as well as by several series of soundings made with 
an iron sounding-rod, shows that the firm superficial layer of the peat is under- 
laid by one of soft, black, peaty mud, of a thickness varying from 1 to 10 or 15 
dm. Beneath this is a layer of firm sand or gravel, with a very uneven upper 
surface, perhaps due to the covering up with the soft muck, of a delta cut by 
many channels. (See fig. 3, p. 111, which gives a north-and-south section at 
1,100 east, as reconstructed from soundings by Professor York and Mr. Paul 
Collins.) The muck below the surface here is, in consistency, much like that at 
the bottom of the harbor, and that underlying the firm surface peat of the 
Spartina border about the barbor. 
The soil of levels above high-water mark differs very greatly on different sides 
of the harbor. On the Spit, e. g., there are large areas where the surface layers, 
at least, are of nearly pure sand, only partially fixed by the tufts of Ammophila 
and occasional clumps of Solidago sempervirens. In depressions near the top 
of the Spit, however, e. g., near 500 east and 800 west, there are patches of firmer 
soil, rich in humus, and supporting a considerable variety of plants. In fact, 
wherever a tree or a group of bushes becomes established on the Spit humus 
accumulates and the soil 1s held together, so that such areas may be left standing 
considerably above the rest of the surface, which is lowered by the removal or 
sand by winds and waves. This is true, e. g., of the area about the Robinia near 
600 west, of that about the group of Ailanthus near 100 east, and of that about 
the group of hus at 540 east. 
The soil above the 8-foot level on the east and west shores of the harbor, 
aside from the gravelly artificial surfaces of the wharves, is not much affected 
by the proximity of the sea. In many places it is springy, wet, and shaded, 
and most of the plants on it are species found in inland wooded swamps, though 
Scirpus americanus does push up the streams to 9 or 10 feet. In drier, sunny 
places, beach-plants, such as Solidago and Atriplex patula hastata, may crowd 
up among the upland forms to as high as the 8.5 or 9 foot level. 
On the Marsh, as we have seen in Section III, the character of the soil and 
the vegetation changes rather gradually in going southward from the middle of 
the Marsh at 8 feet to the foot of the causeway embankment at 9.5 feet. These 
changes in soil and in plant-covering are indicated more precisely in the detailed 
maps of the Marsh (plates x1, xx1, and xx11). The relation of the vegetation 
to the substratum is too complex and the causes of its detailed distribution too 
incompletely understood to make it worth while to take it up in any detail again 
here, after what has been said earlier in this paper. 
In general summary of the relation of the distribution of plants to soils in 
the harbor, it must be considered as evident: (1) That the sparsity of attached 
algee on the bottom of the harbor must be due chiefly to the lack of larger 
particles in the soft mud to which plants like the rockweeds and red alge can 
become attached. Similar tidal basins in the neighborhood, having stony 
bottoms, show a much more varied algal flora (e. g., Center Island and Lloyd’s 
Point). (2) The peaty mud, commonly found between the 1.5 and 6.5 foot 
levels, is dominated by Spratina glabra, with only a subordinate ground-covering 
