494 PRINCIPLES OF STRATIGRAPHY 



in fresh water peats, unless these are subsequently submerged by 

 marine waters. 



Mangrove Marshes. 



These occur in the tropics and take the place of the salt grass 

 marshes of temperate climes. The mangroves comprise some 

 twenty species, characterized by numerous slender trunks varying 

 from five to sixty feet in height. These rise to just above the level 

 of high tide, and are crowned with heavy foliage, which, when the 

 tide is full, appears to float upon the water. From the crown, 

 stolonal processes are sent off and these, in turn, give rise to de- 

 scending roots, which, after reaching the sea-floor, become fixed and 

 serve to support the trees against the effects of the tides and waves. 

 Shaler believes that the mangrove thickets may advance over the 

 sea-floor at the rate of twenty or thirty feet in a century. Their 

 seaward extent is limited by the deepening of the water and the 

 force of the waves, and by fish which feed upon the growing ex- 

 tremities of the roots. The densely interwoven stems and roots of 

 the mangrove form an effective barrier for the retention of silt. 

 The flow of water from the land is checked, and sediment deposited, 

 until the roots and stems are completely buried, when the plants die 

 and give way to other types of vegetation. Commingled with these 

 buried trunks and roots we find a considerable littoral fauna, a 

 part of which was directly attached to the bases of the mangrove 

 or crawled about on them, and another part which lived on or in 

 the mud which accumulated between the stems. (See further, Chap- 

 ter XXVIII.) 



Fresh Water Swamps. 



The sapropelite region of swamps is largely confined to the 

 deeper parts of the lakes where planktonic fresh water algae sink 

 to the bottom upon their death and where Characese grow and pro- 

 duce a layer of sapropelcalcilite or marl. The formation of sapro- 

 pelite is, probably, far less extensive than in marine waters, for 

 terrestrial vegetation will take root in all shallow waters or push 

 out as floating mats from the shore, the deposits resulting being, 

 therefore, chiefly humuliths. Floating algse are rare in all except 

 the smaller, more stagnant, lakes or pools, and thus the chief de- 

 posits of this type on the deeper lake bottoms are the Chara marls. 

 Fresh water swamps may be subdivided into lake swamps, river 

 swamps, and estuarine swamps, which merge into the marine 

 marshes. 



