19-1.] STEVENSON— FORMATION OF COAL BEDS. 599 



The final stage is where the sedge mat closes over the surface 

 and the underlying peat has become firm. Sedge is usually the chief 

 factor in the later stages of lake destruction. At times, the mat is 

 pressed down by the weight of trees growing on it. In one case it 

 was found 6 feet thick, resting on semi-fluid peat. A section at 

 one locality showed 



Feet. Inches. 



1. Sphagnous peat O 6 



2. Moss peat and shrubs 2 o 



3. Moss peat 3 



4. Coarse brown peat, stumps and roots ... 2 6 



5. Remains of shrubs o 2 



6. Dark peat rich in sedge remains 2 



It was impossible to determine the condition farther down as the 

 peat was very wet, but sedges were recognized. Similar conditions 

 were observed in other sections. These all show that the trees w^ere 

 rooted in the mat of pure vegetable material, even when it reposed 

 on the water surface and that, while the trees were growing, the 

 accumtdation of peat was continuotis. 



After the mat has been grounded, Hypiuini hastens outward 

 from the shore, associated occasionally with some Sphagmini. When 

 the surface rises 2 inches above the water level, ferns appear and 

 they are followed by Spliagnum, which persists even when the sur- 

 face is flooded. It is much hardier than Hypiium and, for that 

 reason, it has been regarded as chief factor in the production of 

 peat. Bttt it is often absent, having been found in less than 30 per 

 cent, of the localities examined by Davis. The first tree is the tama- 

 rack, which grows densely on the level of shrubs, but isolated trees 

 are scattered over the open bog. 



Chara-marl occurs frequently in southern ^Michigan but it was 

 not seen anywhere in the northern portion of the state, where the 

 general succession dififers somewhat from that already given. The 

 Chara-stage is wanting ; pond weeds, pond lilies and rushes are of 

 irregular occurrence and the sedge-zone is all important. Owing, 

 probably, to absence of fragments belonging to the higher plants, 

 the work of freshwater alg^e is more apparent than in the southern 



PROC. AMER. PHIL. SOC, L. 202 NN, PRINTED NOV. l6, I9II. 



197 



