496 PRINCIPLES OF STRATIGRAPHY 



marl or sapropelite from floating plants has to occur to bring the 

 bottom to the required level where the higher plants can gain a 

 foothold. These will slowly build up the floor of the lake by partly 

 decayed vegetal matter, the rate increasing with progressive shoal- 

 ing, owing to the increase in the number of plant species and their 

 greater luxuriance. The succession of types is shown in the pre- 

 ceding diagram (Fig. iii), copied from Davis. 



In the filling of lakes by vegetal growth a prominent part is taken 

 by the plants which build floating mats from the shore outward. 

 Chief among these in our temperate climes is the sedge Carex fili- 

 fonnis L., which forms a mat of felted and interwoven stems and 

 roots, making a buoyant structure capable of supporting consider- 

 able weight. This sedge may grow with its rhizomes submerged a 

 foot or more below the surface of the water and its roots extending 

 much farther downward. These mats may be from a few inches 

 to two feet in thickness near their lakeward end, while toward the 

 shore they become more and more solid and eventually no longer 

 float. The solid portion gradually extends lakeward, partly by the 

 accumulation of more or less decayed vegetal matter beneath the 

 .mat, and the pond thus becomes filled. The sedge is sometimes 

 largely replaced by the cat-tail Typha latifolia, which likewise aids 

 in the building of the mat. As the surface of the mat approaches 

 the level of the water, dense grovv^ths of the moss Hypnum will 

 occur; other plants follow, including ferns and the Sphagnum moss, 

 and, finally, the tamaracks and spruces will begin to migrate out- 

 ward, and the pond passes from the quaking bog stage to the more 

 or less wooded tamarack swamp (see Fig. 112). As the forest ad- 

 vances the sedges begin to die, being unal)le to flourish in the shade. 

 The ground becomes covered by a heavy growth of Sphagnum 

 moss, which may grow to some extent in the water, but more com- 

 monly forms a surface growth which may reach a thickness of 

 several feet. The stumps and trunks of dead and fallen trees are 

 gradually embedded in this growing mass of vegetal material, be- 

 coming an integral part of the accumulating peat deposit. As the 

 Sphagnum forms the most conspicuous part of the surface of the 

 swamp and is a vigorously growing plant, it has commonly been 

 credited with being the chief agent in the production of swamp 

 peat, the plant itself being known as the peat moss. It appears, 

 however, that this moss must be regarded as only a contributor 

 so far as swamp peat is concerned. 



In a consideration of lake swamps we must not neglect the fact 

 that lake basins come into existence in a great variety of ways (see 

 ante, p. 116), and that in some cases these basins may from the 



