I9I3.] STEVENSON— FORMATION OF COAL BEDS. 35 



Other members of the series, since mineral charcoal is a common 

 constituent of the brown coals as well as of peat. 



The series is continuous. By slow destructive distillation under 

 pressure all can be converted into anthracite. The coal at Decaze- 

 ville is much given to spontaneous combustion and the operators 

 suffer great loss not only by destruction of the coal but also by con- 

 version of much into a dense brilliant anthracite. The change of 

 brown coal into anthracite by eruptive rocks is a common phenome- 

 non in both Europe and America, so common that anthracite is 

 thought by the great majority of students to be a metamorphic coal. 

 Beside the ordinary coals, which have so many features in com- 

 mon, there are some which might be termed aberrant forms, the 

 cannels, bogheads, kerosene shale ; these, which have been termed 

 sapropelic coals, are minutely laminated, brownish black and have a 

 brownish streak. Ordinarily, they are rich in volatile constituents, 

 which give much more brilliant flame than those from bituminous 

 coal. In mode of occurrence and in some structural features they 

 resemble the organic muds or sapropelites of Potonie, which are 

 found in many ponds and in lakelets within peat swamps. They,, 

 like the other coals, are composed of changed plant material, but they 

 frequently contain animal remains. 



All coals have more or less inorganic material, the ash or incom- 

 bustible portion. At times the quantity is insignificant, less than i 

 per cent, but it often exceeds that of the com'bustible matter, in which 

 case the rock is known not as coal but as carbonaceous or bitumi- 

 nous shale. 



The Extent of Coal Deposits. 



The areas of individual coal deposits vary from a few square 

 yards to many hundreds of square miles. Those of very limited 

 extent are, usually, outlying patches, occupying spaces eroded in 

 older rocks and they abound in some of the western states, where the 

 coal rests unconformably on beds of Mississippian or even greater 

 age. HalF described several in Iowa, most of which consist of 



* James Hall, Rep. Geol. Surv. Iowa, 1858, Vol. I., pp. 121, 124, 126, 130, 

 131, 133; A. H. Worthen, ibid., pp. 212, 223, 234. 



