624 STEVENSON— FORMATION OF COAL BEDS. [November 3, 



Diirntcn, the Schieferkohle rests on a gray-white marl containing 

 Anodonta, ValTata and Pisidinm. That marly clay rests on the Oli- 

 gocene Molasse, which holds a bed of lignite. The woody limbs are 

 still distinct but the rest of the mass has been changed beyond recog- 

 nition. Yet one finds traces of marsh plants in the overlying marls, 

 while underlying the lignite is an undoubted lake-marl containing 

 Unio, Planorhis and Lynmcva. 



Conclusions. — In this presentation of the features characterizing 

 peat deposits, some facts appear in notably bold relief. 



1. Peat deposits vary in form from lenses to sheets; the former 

 are of petty to considerable extent, fill depressions such as pond or 

 small lake basins ; the latter, often of vast extent, originate on ap- 

 proximately level areas, where drainage is imperfect. The bottom 

 and top are apt to be irregular ; the latter because of islands or sandy 

 deposits but especially because of streams and shallow ponds; but 

 the form of the bottom depends on that of the surface on which it 

 rests. The thickness may show great variation ; a few inches of 

 peat at one locality may be continuous with a deposit, 10 or even 

 60 feet thick elsewhere. Great deposits are not continuous verti- 

 cally ; partings divide the bed into benches ; those partings may be 

 very thin, clay or sandy clay with much woody matter, merely desic- 

 cated peat wasted by exposure during a dry period, or they may be 

 sediments, varying irom films of clay to beds of sand, gravel or clay, 

 loose or consolidated. Peat deposits, especially those of great hori- 

 zontal extent, often bifurcate and, at times, the "splits" reunite. 

 The underlying material may be clay or sand — usually clay or marl 

 for the lens-shaped deposits, but very often sand or sandy clay for 

 sheet deposits extending over great areas. Sand with slight admix- 

 ture of clay becomes practically impermeable by absorption of 

 humic acid. 



2. Peat deposits are recognized by macroscopic features as far 

 back as the middle Tertiary. Some of Post-glacial age and several 

 thousands of miles in extent are buried under 3 to 30 feet of sedi- 

 ment ; some Quaternary deposits underlie 30 to 120 feet of trans- 

 ported inorganic matter and the overlying deposits vary from fine 



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