114 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



finds pockets of sand or clay, even of freshwater limestone. The 

 limestone cannot be regarded as extraneous, for in all probability it 

 was formed sur place in ponds within the swamp ; but the other 

 materials are of foreign origin and indicate more or less frequent 

 flooding by detritus-laden water. In considerable areas, the quantity 

 is so great as to render the peat worthless ; in others the material is 

 localized, as in bogs of lake or pond origin, wdiere the peat on the 

 borders is commercially worthless, while midway in the basin it is 

 almost free from mineral admixture. The several benches of a peat 

 deposit often difTer notably in mineral-content, showing variations 

 in conditions during formation. Bones of mammals, shells of fresh- 

 water mollusks, remains of beetles and other insects are of common 

 occurrence. Peat deposits have yielded the best specimens of Pleisto- 

 cene mammalia ; domestic cattle are often mired in swamps and 

 whole troops of armed men have perished in Scottish swamps during 

 flight after battle. 



II. The floor or mur of peat deposits may consist of any mineral 

 material not injurious to plant life. Ordinarily, in swamps originat- 

 ing in ponds, it is composed of more or less nearly impervious stuff, 

 clay, lake marl or Lebertorf mud. Where a deposit has increased 

 by transgression, the mur may be shale or even sand ; but in the latter 

 case the immediate floor is apparently the cover of forest offal, the 

 underlying sand having been rendered more or less nearly impervious 

 by humic acid derived from the organic cover. The characteristic 

 feature of the mur is the presence of roots belonging to peat-making 

 plants. In original localities, where peat was formed in open areas, 

 the roots are those of reeds, rushes water-lillies, etc. — the Rohrricht- 

 boden of German writers being a familiar condition. Where the 

 deposit originated in forests or encroached upon them, one finds in 

 the mur a tangled mass of roots, oaks, conifers, alders, birches and 

 other plants, from which the stems very commonly pass into the peat. 

 These stems rarely extend beyond the peat cover and they are broken 

 off at practically the same level. Where the deposit consists of sev- 

 eral benches, each becomes in turn a mur for the next, and roots are 

 distributed in the peat-mur as in the original clay or other nnir; in 

 each bench the stumps are cut off at or below the top of the peat. 



