The group of marine marshes differs in many important regards from those which are formed in fresh water. Where the tide visits any coast line, and in sheltered positions along that shore, a number of plants, mostly belonging to the group of grasses, species which have become accustomed to having their roots bathed by salt water, begin the formation of a spongy mat, which resembles that composed of _Sphagnum_, only it is much more solid. This mat of the marine marshes soon attains a thickness of a foot or more, the upper or growing surface lying in a position where it is covered for two or three hours at each visit of the tide. Growing rapidly outward from the shore, and having a strength which enables it to resist in a tolerably effective manner waves not more than two or three feet high, this accumulation makes head against the sea. To a certain extent the waves undermine the front of the sheet and break up masses of it, which they distribute over the shallow bottom below the level at which these plants can grow. In this deeper water, also, other marine animals and plants are continually developing, and their remains are added to the accumulations which are ever shallowing the water, thus permitting a further extension of the level, higher-lying marsh. This process continues until the growth has gone as far as the scouring action of the tidal currents will permit. In the end the bay, originally of wide-open water, is only such at high tide. For the greater part of the time it appears as broad savannas, whose brilliant green gives them the aspect of rare fertility.