140 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



such as Salvinia and Trapa; the next layer is full of Arimdo stems 

 with leaves of Salix, evidently blown by the wind. Above this is 

 the coal, composed of Sequoia, Betula and Palmacitcs stems. One 

 sees here the gradual filHng of a freshwater basin, through accu- 

 mulation of in situ vegetation. The abundance of still erect tree 

 stems, some rooted in the floor and others rooted in the coal itself 

 and extending meters into the overlying sand, suggest that all the 

 stems, prostrate as well as erect, are those of an in situ vegetation. 



Naumann^^*^ remarks that stems, prostrate, piled irregularly and 

 compressed, are often enclosed in earthy brown coal. At times, 

 however, erect trees are found, cylindrical and retaining their roots, 

 so that they are where they grew. One finds in these areas that 

 locally all the prostrate stems lie in the same direction, showing that 

 the same force had broken them off and laid them down. He adds 

 a new example of erect trees. Some years prior to publication of 

 his work, the brown coal had been exposed by stripping near 

 Wiirzen in the province of Sachsen ; on the surface of the coal, 

 within a space of about half an acre, he saw between 40 and 50 

 trees, their roots interlocked within the coal bed. 



A mineral, termed pyropissite, occurs at numerous places within 

 Sachsen, sometimes pure but often mingled with ordinary brown 

 coal to form the Schwelkohle, which has been of no little importance 

 as a source of oils. Stohr's^"^ description of conditions, prefacing 

 his discussion of pyropissite, gives some details not recorded by the 

 authors already cited. The strata generally are in no regular order 

 and appear to dovetail ; the brown coal alone appears to be well- 

 defined. The formation is from 30 to 60 meters thick and under- 

 lies 3 to 30 meters of diluvial deposits. The roof of the coal is 

 sand, clay or hard sandstone and there is a similar variation in the 

 floor, though commonly that is plastic clay. The brown coal, where 

 mined, averages about 6 meters, but the thickness varies from a few 

 centimeters to 10, 16 and at one place 20 meters. Owing to irregu- 

 larity in the floor, due to prior erosion, the coal occurs in shallow 



196 Q -p Naumann, " Lehrbuch der Geognosie," 2te Aufl., 1862, Bd. III., 

 p. 204. 



1^' E. Stohr, " Das Pyropissit Vorkommen in den Braunkohle bei Weis- 

 senfels und Zeitz," Ncues Jahrhuch, Jahrg. 1867, pp. 407-409. 



