I9II-] STEVENSON— FORMATION OF COAL BEDS. 613 



E. Shore peat, like transported peat, 25 centimeters, roots of Carex, 

 Equisctum and Phrag)nifes. 



F. Plankton-g>ttja, 40 cm. with remains of infloated Phraginites, 

 Equisctum. etc., some pollen of Picea in upper portion. 



G. Cla}', 50 cm. rich in saltwater diatoms. 



As interpreted by Yon Post, one has here at the bottom, a deposit 

 of plankton tnaterial or Sapropel. It was invaded by the shore peat, 

 on which a forest of birch and alder grew for a short time amid 

 unfavorable conditions, as the swamp was overflowed at times ; this 

 condition became more marked and a sedge swamp followed, in 

 which Sphagnum gradually gained control. Still later, for a short 

 period, during which accumulation of peat continued unchecked, the 

 moor was covered with a dense growth of firs ; but as the moisture 

 increased, the non-water-loving elements disappeared and a Calluna- 

 Eriophoniin moor occupied the area. Sections in Skarby lake com- 

 plex show the same general features as those observed elsewhere in 

 this region. Though there are dififerences in detail, the story is prac- 

 tically the same throughout. The open water deposits, gyttjas rich 

 in plankton material, form the lowest stratum resting on clay or 

 sand ; on this is the shore peat, which gradually passed across the 

 basin. Then came the time of decreasing moisture ; alders advanced 

 on the peat surface, now subject to only occasional overflows ; they 

 were succeeded by birches, which were rooted in the alder peat ; and 

 finally came the great forests of Scotch' fir growing in the birch and 

 alder peat, to be succeeded by Sphagiiitiii-'Hoch.moov peat in the 

 moist Sub-Atlantic stage. Peat-making was continuous in the for- 

 ests and each type of forest peat has its own group of minor plants. 



Buried Peat Deposits. — Some authors have contended that peat 

 deposits on the land are not likely to be preserved because, exposed 

 to air, they must be afifected by atmospheric conditions and eventu- 

 ally must waste away. Under such conditions, it is certain that 

 only such accumulations of vegetable material as are deposited in 

 water-filled basins would be preserved. But the supposed condi- 

 tions are purely hypothetical and are not in accord with those exist- 

 ing in nature. Indeed, one looking at a peat deposit, many feet 

 thick, would have difficulty in conceiving how there could be uni- 

 formity of conditions for a period long enough to permit wastage 



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