36 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



these currents, even in great flood, are powerless against the mat ; 

 the river at some places is diverted into a false channel but at others 

 it passes through a series of shallow lakes. 



The groups of higher plants, contributing to production of peat, 

 are for the most part those which prefer a soil containing organic 

 acids formed during decomposition of vegetable matter. Some of 

 them are provided with root modifications, enabling them to grow 

 even when rooted in water-covered peat. Others, the ordinary 

 conifers and deciduous trees of swamp- areas, have no such modi- 

 fications and grow only on the less moist portions. In case the 

 water-level rise permanently so as to prevent aeration of the roots, 

 the trees die ; but mere accumulation of peat about the roots is not 

 the direct cause of death, as it is proved abundantly by the exist- 

 ence, of mighty trees in the western forests, the intervals between 

 them showing several feet of peaty accumulation, in which young 

 firs and scrubby oaks have grown from the seed. The great Taxo- 

 diuui and Nyssa are rooted directly in the water-covered peat, but 

 aeration is secured by means of the "knees" and the arched roots 

 which rise above the water surface. Aeration is as necessary lo 

 these trees as to the others and they can be drowned quite as easily 

 as the junipers. Lowly forms of plant life make, as a rule, merely 

 incidental contributions to peat, but under certain conditions they 

 may accumulate in mass. Some forms of fresh water algse are con- 

 stituents of organic muds in pools or ponds, which so often become 

 the foundation for peat, while occasionally one finds a layer of 

 diatomaceous earth in or over the peat. 



Classification of Peat. — The great economic importance of peat 

 in some German states led early to close study of that material in 

 all its phases and, of course, to classification, a differentation of the 

 varieties of peat and of the types of deposits. This work had been 

 done in great part by the diggers before scientific students began, so 

 that in all efforts at classification one finds greater or less use made 

 of the popular terms. ZirkeP^ offered a grouping based on the 

 character of the original materials ; 



Moostorf, derived from water-loving mosses, chiefly Sphagnum, 

 23 F. Zirkel, " Lehrbuch der Petrographie," Bonn, 1866, Vol. I., p. 398. 



