46 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



conditions existed, but other growth seems to have been far in excess. 

 Sapropelic deposits, as foundation for ordinary peat, appear to be 

 practically wanting in the United States, as no reference is found in 

 publications by Shaler and Harper, while Davis^^ observed them in 

 only three of the many bogs examined by him in Michigan. The most 

 noteworthy of these is that of the Algal lake ; but the forms in that 

 deposit are no longer thought to be algae, their relations being still 

 undetermined. 



Lebertorf conditions may reappear at almost any time during 

 the history of a peat bog. Sernander-'^ long ago observed the lens- 

 like structure characterizing portions of moors in Sweden, and 

 Weber had called attention to the same feature in northern Ger- 

 many. The study of peat deposits in Narke by the Geological Com- 

 mission led to discovery of the causes and the results were published 

 in 1905. The prevailing opinion had been that shoots of the 

 sphagnum-carpet grow uninterruptedly upward, wdiile the under 

 parts die and are converted into peat. But the sphagnum-tips are 

 killed very easily. On portions of the carpet, where such destruc- 

 tion has taken place, growth ceases, while the surrounding moss 

 continues to grow, so that a depression results. Such depressions 

 are very numerous and are due to various causes. One type, fre- 

 quently observed in Heath- and Waldmoors, is caused by accumula- 

 tion of offal from the plants ; another is caused by surface growth 

 of liverworts or certain forms of algse; while a third comes from 

 fires, footsteps or other accidents. As the surrounding Sphagnum 

 continues to grow, the depressed spots become filled with water ; 

 plants growing on the surface are killed and a Dy-like deposit covers 

 the bottom. The ordinary process of filling follows, sphagnum in- 

 vades the pool and eventually fills it. The depressions sometimes 

 increase by transgression and attain considerable size. An illus- 

 trative section, given by Sernander on his Plate 3, shows that in- 

 terruptions of this kind are of frequent occurrence. Similar de- 

 pressions are familiar in bogs of all sorts. 



In many cases, especially where the water is calcareous, this 



3» C. A. Davis, " Peat," pp. 203, 247, 267. 



39 R. Sernander, " Guide to Excursions," Coiig. Geol. Int., XL, Excur- 

 sion A7, p. 25. 



