I9II.] STEVENSON— FORMATIOX OF COAL BEDS. 97 



In a later paper,^' Potonie says that having supported the cause 

 of autochthony, he must describe a deposit of allochthonous type. 

 The distinctions are simple ; in plants of autochthonous origin, the 

 more tender parts are preserved but they are practically wanting 

 in those of allochthonous origin. In connection with coal beds one 

 has to do chiefly with autochthonous plants ; but in Culm localities 

 he has to do with " Haecksel." shreds of plants, which are char- 

 acteristic of allochthony. These fragments are at time large 

 enough to show by their arrangement the direction of the transport- 

 ing current. 



Allochthonous deposits of carbonaceous material have few 

 botanically recognizable plants ; many stems and branches, often 

 coal coated but with surface sculpture so obscured that determina- 

 tion is impossible ; stems of the Knorria type are of frequent occur- 

 rence ; while Stiguiaria is almost wholly absent, those which do occur 

 being imperfect. Sub-surface organs can be carried away only 

 after having been washed out from their place : other portions of 

 plants nuist be the essential material of a transported mass. 



He presents the following contrasts. 



Autochthony. Allochthony. 



1. Coal beds common. i. Coal beds are. 



2. Haecksel deposits absent or 2. Plant remains prevailingly 



insignificant. Haecksel. 



3. Determinable plants numer- 3. Few determinable plants ; if 



ous, especially in roof. coal bed, Haecksel in the 



4. Few indeterminable casts. roof. 



5. Knorria rare. 4. Indeterminate casts abundant. 



6. Abundant Stigmaria in the 5. Knorria abundant. 



liegend. With their appen- 6. Stigmaria absent or rare ; they 

 dices. are without appendices. 



7. Excellent preservation o f 



ferns. 



Potonie presented a brief systematic discussion of the whole 



"' H. Potonie, " Die Merkmale allochthoner palaeozoischer Pflanzen-Abla- 

 gerungen," Xatumnss. Wochenschrift, XIV., 1899, pp. 81, 82. 



PROC. AMER. PHIL. SOC, L, I98G, PRINTED APRIL 26, I9II. 



97 



