I9II.] STEVENSON— FORMATION OF COAL BEDS. 113 



that under exceedingly favorable conditions a peat bog has gained 

 one foot of thickness in five years but that in one case this increase 

 appeared to be only one foot in two hundred years. With the con- 

 ditions normal, the rate of increase seems to be not far from one 

 foot in ten years. Reasoning from the approximately ascertained 

 ratio of volume of peat and the resulting coal, he conceives that 300 

 years would be required for the formation of one foot of coal, thus 

 giving a period of about 4.000 years for accumulation of the Pitts- 

 burgh coal bed in western ^Maryland. The minimum period to be 

 assigned for formation of the 300 feet of coal in the Appalachian 

 basin is not far from 100.000 years. 



In his later paper, seeking to ascertain whether or not a coal 

 bed may be utilized as a time measure, he indicates some complexities 

 of the problem, one of which is important. A coal bed, 18 inches 

 thick at one locality may be 15 feet at another, the latter thickness 

 requiring for accumulation 4,000 years more than the other. As the 

 rocks accompanying the thinner bed show no compensating differ- 

 ences, the 18 inches is all that was formed while the 15 feet was 

 accumulating elsewhere. There was either slow growth or a time- 

 break, that is a period of no deposition, before or after deposition 

 of the thin bed. 



"' Smooth-partings " are evidences of time-breaks and represent 

 locally nonconformity between the under- and the overlying beds : a 

 " smooth-parting" at one place may be equivalent to 40 feet of shale 

 at another ; an inch or two of cannel may have similar equivalence. 

 Slow growth and temporary cessation of deposition are important 

 elements of the problem. 



Dannenberg^'"* finds strong arguments in favor of autochthonous 

 formation in the vast extent of some coal areas, the presence of 

 the tenderest plant-parts in coal inclusions, the abundant occurrence 

 of roots directly under tlie coal, and the identity of coal-forming 

 plant species with those found in the enclosing shale rocks. Not all 

 localities show these features with equal clearness, for in some cases 

 there are variations along dip and strike like those in delta deposits, 



'"^ A. Dannenberg, "Geologic der Steinkohlenlager, Berlin, 1909; Erster 

 Tell, 197 pp. The citations are from pp. 18-27. 



PROC. AMER. PHIL. SOC, L. I98H, PRINTED APRIL 25, IQII. 



113 



