112 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



ties ; even small deposits show pauses like those which characterize 

 those of greater extent. Many times a cyclical order is distinct and 

 the deposit is divided into benches. The process was interrupted 

 again and again, so that the surface became dry enough for growth 

 of trees, not merely of conifers but, in some cases, even of deciduous 

 trees of forest type. Sometimes such pauses were of long duration 

 as is shown by the age of the trees. The forest growth was fre- 

 quently very dense, for the peat is loaded with stumps and broken 

 stems. A considerable proportion of the trees were overturned, per- 

 haps by the wind, and sank undecayed into the soft pulp. The 

 moister conditions returned, the trees were drowned and peat growth 

 was resumed. This succession may be repeated several times in a 

 single deposit. 



The benches may pass gradually, the one into the other, or they 

 may be defined sharply by partings. At times the parting consists of 

 Torffaserkohle or mineral charcoal, mingled with extremely fine 

 mineral matter, the residuum on the surface of peat long exposed to 

 oxidation. Such partings mark a period of dryness without invasion 

 by forest, during which the peat wasted. But partings of clay, sand 

 or marl mark invasion by water carrying detritus. After a period, 

 long or short, the surface is again covered with shallow water and 

 peat making is resumed, the parting serving as mur for the new 

 accumulation. The parting of Torfifaserkohle and ash may be con- 

 tinuous with the thick parting of transported material, so that when 

 peat making has been resumed over the latter, the process would 

 extend over the thin parting of wasted peat. It is important to bear 

 in mind that the thin parting is more than equivalent in length of 

 time to the thick parting. The new peat, expanding by transgression, 

 required a period for advance; the peat underlying the parting of 

 transported material may be strictly contemporaneous throughout, 

 except that the upper part is represented by the thin parting. A 

 thick cover of detrital matter may bring accumulation finally to a 

 close in one portion of an area while growth continues in another. 

 Sand dunes in some localities within the United States have covered 

 bogs of small size so that no farther increase was possible ; but in 

 the Baltic region, as shown by German and Scandinavian geologists, 



