330 



seam is overlapped, the overlap is to the northeast or 

 north. 



These features are illustrated by the accompanying 

 sections which are drawn to scale. 



It appears from the evidence given above as if the 

 variation was due in large part to a folding which affected 

 the coal seams when the clean coal was in a fairly plastic 

 condition. This conclusion is especially well substantiated 

 in another part of the Wellington seam, where it is composed 

 of several sub-seams separated by dirty slickensided coal 

 or rash. During the deposition of the seam, conditions in 

 which fairly clean carbonaceous matter was deposited 

 must have alternated with those during which the car- 

 bonaceous matter was deposited with a large amount of 

 silt. When the seam was folded, the clean coal was 

 apparently forced away from the tight bends, where the 

 folding caused an increase in the vertical pressure, and 

 left the seam at these places composed almost entirely of 

 rash. The clean coal flowed to where there was a 

 corresponding relief of vertical pressure forming a swell 

 where the seam, except for the rash at the top and bottom, 

 consists chiefly of clean bright coal. 



Besides the barren places or wants due to folding subse- 

 quent to the deposition of the seam, there are large wants 

 due solely to silting, for in some instances the silting must 

 have persisted throughout the period of coal formation. 

 Also large and persistent partings of shale occur between 

 the sub-seams. 



Both types of variation occur in the Douglas seam. The 

 seam varies from nothing to 30 feet (10 m.) in thickness, 

 and averages about five feet (1-5 m.) although over large 

 areas the average thickness of the mineable coal is between 

 three and four feet (-9 and 1-2 m.). The floor of the 

 Douglas seam is usually a rather weak sandy shale, and the 

 roof, although stronger, is very variable, ranging from a 

 sandy shale to a fine grained conglomerate, the principal 

 type being a shaly sandstone with sandstone layers and 

 lenses of fine grained conglomerate. Unlike the conditions 

 in the Wellington seam the pinches and swells are caused 

 chiefly by irregularities in the floor, the roof being fairly 

 smooth. At the pinches the seam is composed almost 

 entirely of rash, like that of the Wellington seam, although 

 as a rule it is harder. The coal occurring in the swell has a 

 compact texture, but rather dull lustre. It is irregu- 



