[PENHALLOWI DEVELOPMENT OF CERTAIN MARSH LANDS 17 
plants of the marsh, and at the same time provide for the addition of 
that slight amount of silt which is always found to be a constituent of 
the marsh turf. 
A fourth feature of salt marshes is to be found in the character of 
their shore-line which generally consists of an outcrop of ledge rising 
to a height of from one to many feet above the marsh, and bearing a 
mixed forest growth. Within the area of the marsh itself, masses of 
ledge, often of considerable extent, may project above the general 
surface and thus form islands which usually bear a vegetation like that 
of the mainland, though in some cases the number of species or genera 
may be so restricted as to form an isolated flora of essentially one genus. 
All these features go to show that the marsh was formed in a relatively 
shallow basin or pocket of the general rock formation, and they offer 
very strong suggestions as to the possible method of development. 
Finally, the accumulation of vast quantities of plant remains, and 
often also, of animal remains, especially of marine mollusca, is 
inseparable from the formation of the marsh itself. The peculiar 
conditions under which such organic remains are buried and brought 
under the operation of decay, results in the formation of large quantities 
of gas which is liberated continually from the surface layers and so 
passes off into the air in an unrecognized form, or it may be accumu- 
lated in the soil itself where it is sometimes held in small pockets and 
retained there for an indefinite period. The final liberation of these 
gases may be attended by peculiar phenomena of a very striking 
character, but ordinarily they pass off in such small quantities that 
attention is not directed to the fact beyond a recognition of the 
accompanying fcetid odour, a comment upon the liberation of bubbles 
which escape from the bottom and rise through the over-lying water, 
or a passing expression of wonder at the dim and elusive light of the 
more rarely observed “ Jack-o-Lantern.” 
In discussing the method of formation of salt marshes, Shaler (6) 
considers those cases in which the opening in the barrier beach has been 
established with the formation of an interior lagoon flooded by salt 
water at every high tide. As his description has an important bearing 
upon the marshes now under discussion, it will be profitable to give his 
own words. His preliminary statement shows that “barrier beaches 
formed by the action of the waves, cut off interior areas which, being 
fed by fresh water streams, soon become converted into lagoons. The 
lagoon or bay between the barrier beach and the mainland, being origin- 
ally shallower, and receiving accessions of detrital matter from the 
rivers, from the sand blown over the beach, and from accumulations of 
organic remains, is gradually brought into the condition of a swamp 
