114 STEVEXSOX— FORMATION OF COAL BEDS. [April iS, 



there are no erect trees in the deep water roofs of Group B, where 

 only broken plant fragments were seen : whereas they are found in 

 the, at most, shallow-water roofs of Group A, where leaves occur, 

 in situ, spread out flat and intact. In the collieries at Lens. 19 roofs 

 with plant leaves /;/ situ have erect stems : 7 of these roofs contain, 

 elsewhere, fragmentary plant remains and lacustrine shells, but not 

 one of these contain such remains in the localities where they have 

 erect trees. All the erect trees were found in roofs of the Group A 

 type; not one was found in any roof which is persistently of the B 

 t}-pe ; 28 such roofs exist in the Lens area and all were studied. At 

 Lievan, 7 roofs of the A type have yielded erect stems, but none has 

 been discovered in any of the 17 roofs belonging to the B type. 



These detailed studies, made in small areas where the conditions 

 are apparent, confirm the opinion of Dawson based upon study of 

 the Acadian outcrops and fully justify the conclusion reached by 

 Barrois ; erect trees are not found in deposits laid down in water 

 deep enough to permit floatation ; they are found only in deposits on 

 which there was never more than a shallow cover of water. 



Irregularities ix the Roof. 



In all coal beds there are what the miner calls " troubles." Some 

 of these, such as " clay veins " for the most part, are due to disturb- 

 ance after the column had been deposited, as they pass into overlying 

 rocks ; otliers, irregularities of tlie bottom, were due. ordinarily, to 

 the uneven surface on which the coal accumulated ; but there are 

 many which mark the courses of streams which continued after 

 accumulation of vegetable matter had begim and were obliterated 

 slowly by encroaching plants. 



Irregularities in the roof are generally much more perplexing 

 tlian those in tlie floor. They are the " washouts." to which refer- 

 ence is made in almost every work on coal fields, and they are closely 

 related to the greater " washouts " or filled valleys. In those to be 

 considered here, only the coal and its roof are concerned. ]^Iore 

 or less of the coal has disappeared and occasionally the apparent 

 '•eplacement extends even to the underclay. Seen in cross-section. 



