STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. Ill 



persists even after destruction of the other portions, so that the 

 flattened bark becomes, as it were, an imprint on the pulp. 



6. The stages of growth in peat deposits depend very largely on 

 the original topography of the area. In the filling of water-basins, 

 the first stage is formation of mud on the bottom. This may be cal- 

 careous, formed by Chara and mollusks, and may hold remains of 

 water animals, pollen, spores, freshwater algse and vegetable remains 

 of other sorts, floated in by streams or blown in by the wind. If 

 streams carry detritus and the water have low calcareous content, 

 the bottom is covered eventually by fine silt with similar organic con- 

 tent; but if the pond be free from influx of detritus and calcium 

 salts, the first deposit is a mud consisting of remains of aquatic 

 animals, freshwater algae with spores and pollen blown in by the 

 wind. This is the Lebertorf stage, the Sapropel stage of Potonie. 

 This material, in some cases, increases with great rapidity, and the 

 water, at length, becomes so shallow that certain types of aquatic 

 plants take root and the formation of normal peat begins ; first, the 

 plants rooting under several feet of water ; then reeds encroach and 

 the rushes and sedges advance to form a floating cover, on which 

 shrubs and even trees take root along with ferns and, in man}^ locali- 

 ties, eventually Sphagnum. The trees advance, conifers first, to be 

 followed by deciduous forms of the forest type when the surface be- 

 comes dry to a depth of a foot or more. Throughout, one finds abun- 

 dance of spores and pollen grains, and occasionally a lens of Lebertorf 

 occurs, marking the site of a pool or pond in the growing mass. This, 

 in a general way, is the succession as worked out by the early ob- 

 servers and confirmed by all students during the last third of a 

 century. 



The succession may dift'er somewhat in deposits formed on plains 

 bordering great rivers or on coastal areas. These begin frequently, 

 perhaps generally, in small, shallow ponds, caused by local obstruc- 

 tion of drainage and expanded by transgression, which led to union 

 of many small deposits. The Lebertorf stage could exist in the 

 original small spaces but not in the newer portions formed during 

 transgression, except where local ponds originated in the peat. 



7. The accumulation of peat has been continuous in few locali- 



