191 1.] STEVENSON— FORMATION OF COAL BEDS. 625 



clay with plant impressions to fine or coarse sandstone, conglomerate 

 or even breccia. 



3. Many vast moors, such as those of the Netherlands and North 

 Germany as well as great and small moors in the United States and 

 elsewhere, are at only a few feet above tide. A very slight depres- 

 sion suffices to bring the surface below that level and to introduce 

 marine conditions. In lowdand areas, thousands of square miles in 

 extent, one finds a marine deposit, with characteristic fossils, imme- 

 diately overlying peat, which is sometimes continuous with a still 

 living moor above high tide. In such areas, one finds occasionally 

 a marine deposit, clay or sand, immediately underlying the peat. 

 The overlying or the underlying material or both of them may be 

 distinctly calcareous. 



4. The passage from peat to the overlying deposit may be abrupt 

 or it may be gradual through alternations of peat and sediment. 



5. The channel ways of streams crossing the moors are traceable 

 in borings after the moors have been covered with sediment ; they 

 contain little or no peat. 



6. The peat deposit is not always homogeneous. Sapropel, or- 

 ganic mud, is the foundation in a great proportion of lake deposits 

 in Europe and in some within the United States ; it is probably 

 absent at bottom of great sheet deposits ; but it may occur as lenses 

 in any part of the section, marking the sites of shallow ponds. 

 Sapropel is an unimportant constituent of true peat, which is pro- 

 duced by water-loving land plants, the work of other types being a 

 negligible factor. The several benches of a deposit may difl:'er nota- 

 bly in structure and composition. Peats are laminated even when 

 new, but under compression, the lamination is characteristic and the 

 material has a coal-like appearance. 



7. Peat varies greatly in purity. At times, it has less ash than 

 is found in plants whence it is derived, owing to the action of 

 organic acids on silica and other mineral constituents ; in most cases 

 it shows notable variations, both vertically and horizontally, that 

 variation depending chiefly on extent of exposure to flooding by 

 muddy waters. Peat often contains a considerable quantity of iron 

 and calcium in combination with carbonic, sulphuric and phosphoric 



223 



