98 STEVENSON— FORMATION OF COAL BEDS. [April 21. 



subject in 1905 ;''^ since that time, in successive editions, he has 

 widened the scope of his inquiries until, in the fifth, the presentation 

 covers every phase with abundant illustration from German areas 

 and references to those of other lands. Only certain portions of the 

 work can be referred to in this place but, farther on, many citations 

 will be made. He approaches the subject from the double stand- 

 point of stratigraphy and palaeobotany. 



The coals and allied substances are termed Kaustobiolithe, be- 

 cause they are combustible rocks of organic origin. He groups 

 them into — • 

 Sapropel deposits, originally " stinking muds " composed of aquatic 



animals and plants. 

 Humus deposits, derived from land plants. 



The former include the cannels, the oil shales and, as a derived 

 product, petroleum ; the latter include the ordinary brown and stone 

 coals. The difference in origin of the two groups is evident from 

 the physical composition shown by the microscope as well as by the 

 chemical composition, the Sapropels yielding compounds of the para- 

 ffine group while humus deposits yield compounds of the benzol 

 group. The Sapropels are formed in quiet, almost or wholly stag- 

 nant water and are of limited extent ; whereas the humus deposits 

 were formed as are the moors of to-day and are of vast extent. 

 He illustrates the modes of origin by description of a great bog in 

 northern Germany, which exhibits the passage from sapropel muds 

 at its shore, to the Flachmoor, well wooded ; thence by the Zwisch- 

 enmoor, with changing type of trees, to the Hochmoor, hour-glass in 

 form, which is treeless except alongside of rivulets. He compares 

 the conditions with those existing in sapropel and humus deposits of 

 the older periods. The existence of both autochthonous and alloch- 

 thonous deposits is recognized, but he asserts that the former have 

 been the prevailing type throughout and that, in every age, the 

 latter have played an insignificant part. 



Potonie finds a strong argument for autochthony in the surpris- 

 ing resemblances, chemical and ]^lnsical, existing between beds of 



"" H. Potonie, " Die Enstehung dcr Stcinkolile," Matimciss. IVochcnsclirift, 

 IV., 1005, pp. 1-12: tlic latest edition i,s "Die En.stehung der Steinkohle und 

 der Kanstoliiolithe iibcrliaupt," fnnfste Aufl., Berlin, 1910, pp. 225. 



*J8 



