118 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



other conditions, the bog may be covered by mineral materials, as 

 on the lowland of Holland, Belgium and France, and waste by 

 oxidation is prevented. By one method or the other the peat is 

 preserved indefinitely : by the former method, the continued increase 

 of peat is assured in most cases, as the forested surface again be- 

 comes marshy and peat production is resumed, to end again in a 

 forest. This cycle has been reported again and again from peat 

 bogs of northern Europe. The thickness of a deposit depends, other 

 things being equal, upon the period of growth ; the thickest deposits 

 reported are those in Alaska and in tropical and subtropical regions ; 

 in those regions climatal changes have been comparatively small 

 since the Quaternary began. 



i6. The composition of peat depends ordinarily upon its age, 

 that at the bottom of a deposit not only approaches complete disin- 

 tegration, so that to the unaided eye it shows no trace of organic 

 structure, but it also is far advanced in carbon-enrichment. Yet 

 peat from neighboring localities, where conditions seem to have been 

 similar, may show great dissimilarity in composition ; one finds 

 strange contrasts even in the benches of a single deposit, for some 

 may be far advanced while others consist of almost unchanged 

 plants. This study is not concerned with the processes involved in 

 conversion of vegetable matter, so that one must be content with the 

 statement that some benches were buried when those processes had 

 been checked at an early stage and that apparently no progress has 

 been made since burial. The carbon in peat may vary from little 

 more than 40 per cent, in the topmost layer to 49 in the next — the 

 first used for fuel — and to 64 in the bottom portions. But in bogs 

 where the surface growth has ceased or has been unimportant for a 

 long period, the part immediately below the surface may have 58 

 to 60 per cent. Oxygen shows similar variation ; there being 36 to 

 40 per cent, in the highest part used for fuel while the oldest, 

 densest portions may have only 26 or 27 per cent. The ordinary 

 fuel peat has from 57 to 64 per cent, of carbon. Density is not 

 evidence of advanced change ; the dense, hard, black Schieferkohle 

 of Utznach, compressed by the heavy cover, has 56 per cent, of 

 carbon and 36 of oxygen. 



