108 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



by transgression. These, though continuous superficially, are not 

 strictly contemporaneous throughout. The buried deposits of 

 Holland-Belgium-France are continuous with living bogs on the 

 mainland ; but the buried peat, in greatest part, is older than that 

 now exposed, as evidently the marsh crept gradually inland during 

 the subsidence. In like manner, the great deposits formed on 

 plains show notable variation in thickness as well as in composition. 

 The vast peat-covered plains of Alaska and Siberia have a con- 

 temporaneous top layer, but the underlying portions of the deposits 

 are probably very far from being strictly contemporaneous. 



3. The condition prerequisite to formation of peat is an abundant 

 supply of moisture with sluggish drainage ; this does not mean that 

 alternating wet and dry seasons are necessarily preventive. If the 

 supply of moisture suffice to keep the main mass moist, the loss 

 during the dry season is more than made good by growth during 

 the wet season, as shown by some tropical swamps. This condition 

 of moisture depends greatly upon the topography, which determines 

 the character and extent of drainage. Temperature is important as 

 affecting rapidity of growth. In Spitzbergen, 78 degrees, North 

 Latitude, peat covers considerable areas but the deposits are very 

 thin, for the season of growth is brief and the temperature is always 

 low. But the growth is much greater in the Alaska region to 

 beyond the Yukon, where the plane of perpetual frost is within 6 to 

 14 inches from the surface. The atmospheric temperature during 

 the summer is higher and the winter temperature is lower than in 

 Spitzbergen as the climate is continental, not insular. In the cold 

 regions, decomposition is less advanced than in lower latitudes and 

 the accumulation is of vegetable matter rather than of peat, properly 

 so-called. In the tropical areas, where the topography permits 

 proper moisture conditions, it is evident that prolonged high tem- 

 perature in no wise prevents accumulation but rather encourages 

 it by favoring rapidity and density of vegetable growth. Koorders, 

 Molengraaff, Harrison, Kuntze and others have described the vast 

 deposits in tropical East Indies and South America. Harper, Hil- 

 gard, Lyell and others have described subtropical peat deposits in 

 the United States, while manv observers have written about the 



