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



cessively. Among other illustrations, he refers to the discovery at 

 Rotterdam of two bogs, 5 and 6 meters thick, separated by 4 meters 

 of clay; to the presence of erect trees which, despite the long period 

 which has passed since they sank below the water surface, are still 

 standing on the sea bottom, partly surrounded by sediments ; such 

 trees on the coast of the islands of Sylt and Romo are of types which 

 disappeared from that region many hundreds of years ago. 



Changes in grade of rivers, caused by damming or by crustal 

 movements, would lead to covering of bogs with sand or mud and 

 to the accumulation of rock masses. He finds confirmation of this 

 view in Livingstone's statements respecting the floods of African 

 rivers and in the observations of others elsewhere. 



Lesley'^' in prefaces to reports by geologists of the Pennsylvania 

 survey, made frequent references to hypotheses respecting formation 

 of coal beds. Ordinarily, he preferred to present the matter, as it 

 were judicially, giving the difiiculties in the way of accepting the 

 hypotheses and leaving the decision to the reader. lUit in two of 

 the prefaces he offers some important suggestions. 



W. G. Piatt described a little basin, barely a mile and a half 

 across, in which three sections of Coal bed D were obtained. In 

 all of them, the bottom bench is 2 feet 7 inches thick and composed 

 of brilliant coal ; but the upper part is a dull cannel or cannel shale, 

 measuring i foot 3 inches, 8 feet 3 inches and i foot 2 inches, while 

 between the last two the dip is about 8 degrees compared with about 

 one degree elsewhere. A noteworthy feature is that while the ash 

 in the cannel is from 21 to 25 per cent, and that in the pure coal 

 below is only 1.6 per cent., yet the ratio of volatile matter to fixed 

 carbon is practically the same throughout. 



Lesley felt convinced that the petty basins, in which cannel was 

 deposited, were waterways or pools and that more of them existed 

 at once in certain horizons than in dthcrs. They were not due to 

 erosion for the underlying coal bed is not cut out, it is merely de- 

 pressed. There is no evidence of currents, for the mud is fine, the 

 lamination perfect and the roof soft. The pools were almost stag- 

 nant. How could a depression come about to give, as here, a dip 



" J. P. Lesley, Second Geol. Survey of Pennsylvania, Indiana County, 

 1878, pp. xiv-xviii ; Lawrence County, pp. xix-xx. 



50 



