196 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



action. A layer of mineral charcoal and minutely divided inorganic 

 matter may be the residuum from a considerable thickness of peat ; 

 the proportion converted into mineral charcoal and so rendered prac- 

 tically indestructible would be small compared with that wasted by 

 oxidation and removed by the wind. 



Evidence of long-continued interruption of ordinary peat forma- 

 tion is afforded by forest beds within the coal deposit. At Sen f ten- 

 berg, as shown by Potonie, the surface of the bog became dry enough, 

 more than once, to permit growth of trees; in the Cologne-Linz area, 

 the dryness was such and the period so long that a forest, containing 

 trees with i,6oo annual rings, had full opportunity for development. 

 In much of the Cologne region, the vast proportion of the stems 

 are prostrate, but that condition is by no means evidence that they 

 are in any but the place of growth. They are merely overturned 

 trees as are thoise of the white cedar swamps of New^ Jersey or those 

 in the cypress swamps of Florida or the bogs of Borneo. One layer 

 in the Cologne-Linz district is a meter thick but erect stumps are 

 present among the prostrate stems, as Horner and Heusler have 

 shown. Trees of such immense size as those described by Horner 

 indicate a very prolonged period of comparative dryness, during 

 which the peat, as soil for the trees, was protected from wasting by 

 offal dropping from the dense forest cover. A somewhat similar 

 condition was noted by Wegemann in the Barber coal field of 

 Wyoming. 



The floor of coal beds is as variable as the roof, but it is usually 

 clay or marl. The transition from the coal is occasionally abrupt, 

 but in most cases the passage is gradual, through a faux-mur, con- 

 sisting of alternating layers of coaly material and the mur rock. 

 Very frequently the mur contains fresh water shells, that condition 

 being reported from many places in all parts of the wofld. Land 

 shells are not rare ; they may have been floated in or they may mark 

 drier places in the swamp. Leaves are found frequently in the marls 

 and clays. The notable feature of the mur is the presence of roots 

 attached to stems projecting into the overlying coal. At times the 

 roots alone remain recognizable, the stems having been merged in 

 the coal. Usually these are those of swamp types; but in cases 



