STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 49 



of which tamarack {Larix laricina) and arbor vitte (Thuja occi- 

 dentalis) are among the first to invade the surface. When bog 

 conditions have disappeared and the surface has become covered 

 with mould, deciduous trees advance and crowd out the conifers. 

 As the mould-cover is very thin and the wet peat is reached very 

 quickly, the roots of this forest group rarely extend downward more 

 than a foot, but they spread out in all directions, as do those of the 

 conifers, thus giving stability. 



Structure and Constituents of Peat Deposits. — An intimate ex- 

 amination of a peat deposit leads to conviction that the process of 

 accumulation may not be so simple as is indicated in the preceding 

 generalized paragraphs. There are many modifying conditions. 



A peat bog shows, speaking generally, a thin layer of living 

 plants on top, under which the vegetable matter becomes more and 

 more disintegrated downward until, toward the bottom, the greatest 

 part shows no vegetable structure to the iinaided eye and the mass 

 consists of felted stuff cemented by a humus-like substance. This 

 cement is removable easily by weak solution of caustic potash and 

 the dried residue tends to fall to a powder. The change downward 

 is not, however, always in increasing ratio. 



A peat deposit is rarely continuous from bottom to top, but is 

 commonly divided at irregular distances by partings of one sort or 

 another. These may be thin, consisting of finely divided mineral 

 matter holding a charcoal-like substance, Torft'aserkohle of v. 

 Gitmbel, or they may be thicker and composed of sand, clay or 

 other transported matter. The thickness of individual partings 

 varies greatly, so that the intervals between the several benches 

 of the deposit may increase or decrease. Lorie's''^ observations in 

 Holland, Belgium and north France make this variation sufficiently 

 evident. The benches themselves differ in peculiarities of the peat 

 and in character of the ash, as one would suppose in view of the 

 different plant-associations marking the several stages of growth. 



The opinion has been expressed that a layer of organic material 

 is essential as prerequisite to formation of a peat deposit, and the 

 assertion has been made that, in any event, a Hochmoor with its 



*3 J. Lorie, " Les dunes interieures," etc.. Arch. Mus. Teylcr, II., Vol. III., 

 pp. 424-427. 444- 



