598 STEVENSON— FORMATION OF COAL BEDS. [November 3. 



Peat deposits fill depressions but, in some cases, are formed on 

 almost level areas. Depressions more than 25 feet deep may be 

 filled by alg^e, by floating species of seed-bearing plants, by sedi- 

 mentation, by plant growth from the sides or by a combination of 

 these processes. A frequent succession is Chara-marl, on which 

 rests a peaty soil in which plants take root ; the land marsh moves 

 out and tamarack advances on the deeper peat of the shore. As 

 the water becomes shallower, each shore type moves out and is suc- 

 ceeded by the type behind — the water growing warmer and more 

 aerated. Formation of peat on a flat space is much under the same 

 conditions as those on the surface of a filled depression. When the 

 drainage is poor, liverworts or some mosses take possession; if not 

 too wet, rushes, sedges and grasses appear. Accumulation makes 

 the place wetter and only the hardier plants remain. Sedges are 

 the chief peat-producers under these conditions. 



The process of filling a depression is often very complicated. 

 In southern Michigan, the early stages are shown in many lakes, 

 which are surrounded by zones of aquatic plants. More or less 

 detritus, organic and inorganic, finds its way into the lake. Where 

 the process is more advanced one can trace the whole succession. 



The lowest deposit is formed of Cliara and floating algc-e. This 

 is succeeded in the shallower water by the Potamogcton zone and 

 that bv the water lilies. Just beyond this one comes to the floating 

 mat of sedges, extending on the water surface to a considerable 

 distance from the shore and buoyant enough to support a consider- 

 able weight. The earlier stages may provide soil for rooting of the 

 sedges at the shore line, but the mat itself is wholly unsupported for 

 a considerable distance and is often 18 inches thick. Finely divided 

 material from the undersurface of the mat increases toward the 

 shore, where it becomes dense and the mat is no longer floating. 

 Thus is built the solid peat, structureless, decomposed and nearly 

 black. The surface rises gradually after grounding of the mat and, 

 at each level, new plants appear. Shrubs and Sphagiinni advance 

 to be overcome in turn by tamarack and spruce, which in their turn 

 are overcome by the marginal flora from behind. Tamarack accom- 

 panied by ferns grows far out on the bog. 



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