STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 47 



plankton deposit of bacteria, algse and especially pollen and spores 

 is concealed in the accumulation of marl from Chara and mollusks. 

 At best, it is characteristic only of filled water-basins ; it occurs 

 rarely in the great deposits originating on broad coastal or river 

 plains. This is not to assert the total absence of such material. 

 Those great deposits frequently were due to union of numerous 

 smaller ones, each of which filled a depression of moderate extent 

 and afterward expanded by transgression on the plain. Lebertorf 

 may form the floor in the original depressions, though it may be 

 thin, owing to rapid invasion by the plants giving normal peat. In 

 the other condition, where swamps were caused by obstructed 

 drainage, Lebertorf-forming agencies no doubt existed, but they did 

 not predominate. Indeed, those agencies are always present, except 

 during periods of interrupted growth, due to dryness, as one may 

 learn by descriptions of almost all bogs, which show that freshwater 

 algse along with pollen and spores are accessory constituents at all 

 horizons. Whenever a pond is formed in any considerable deposit, 

 such as Dismal Swamp, originally covering not less than 1,500 

 square miles, the conditions are prepared for formation of a Leber- 

 torf lens. 



The Succeeding Stages Vary. — The earlier studies were made 

 almost wholly upon peat deposits filling former water-basins, which 

 had escaped covering and which had had a, so to say, continuous 

 history from a very early period. When the process of filling was 

 uninterrupted save by variations in temperature or moisture, the 

 normal succession may be shown by the bogs in a great area, as in 

 most of Sweden, north Germany and the British Isles. But where 

 the origin was difl:'erent, a wholly dissimilar section may be found. 

 Geikie, in describing the moors of Scotland, says that they often 

 mark the sites of lakes and ponds, but at times they cover the ruins 

 of ancient forests. When the forest was overthrown, drainage was 

 intercepted, stagnant swamps were formed and water-mosses took 

 root. He refers to several illustrative instances. In the Forest of 

 Mar, large trunks of Scotch fir, which fell from age and decay, were 

 soon immured in peat, formed partly from decay of their perishing 

 leaves and branches and partly from the growth of Sphagnum and 



