STEVENSON, COAL BASIN OF COMMENTRY 
169 
10. Sandy clay. 
Only the surface was seen and the thickness could not be ascer¬ 
tained. This surface is irregular, hummocky, with numerous 
saucer-like depressions, several feet across and filled with coal. 
At one place this north wall shows a great step in the clay, as 
though the material, before consolidation, had been piled up against 
some obstacle. The clay contains vast quantities of carbonized 
plant remains but no fragments of Stigmaria were observed in a 
space of 20 by 30 feet. Underclay of the ordinary type is wanting 
here. 
Leaving the trench by a stairway on the southerly side, one finds no trace 
of the Saint-Edmond sandstones, though they are present at the extreme 
west end. Instead there are mostly more or less sandy, fine-grained shales 
with occasional bands of fine-grained sandstone, in which are rounded 
pebbles of shale, carbonaceous shale and even of coal. At 85 feet, by barom¬ 
eter, above the Grande Couche, one finds 
Sandstone, 8 to 10 feet 
which has many streaks of coal as well as rounded pebbles of shale and coal. 
This sandstone is not reached in Saint-Edmond, though the wall there rises 
to 150 feet above the coal bed, but it is shown in the Tranchee de Goutilloux, 
at a little way southward. In Foret, the interval, measured without regard 
to the dip, is less than half that at the west end of Saint-Edmond, so that 
the thinning in this direction is not confined to the sandstones seen there. 
This highest sandstone of Foret is the lower portion of the Gres Noirs group, 
which becomes important along the eastern outcrop. It is very irregular 
in Foret and at times it shows pots of shale with almost concretionary struc¬ 
ture. One of these has a bit of sandstone as the core. 
Just below the Gres Noirs, the wall shows a feature, often observed else¬ 
where in a fragmentary way, but here exposed for a hundred feet or more. 
The sandstone rests on a bed of shale and both describe some gentle flexures; 
next below is a bed of shale, whose upper surface accords with the flexures, 
but the lower surface as shown in the wall is straight and rests on a bed of 
alternating thin shales and sandstones which has been pushed into many 
petty folds, not shared in by either overlying or underlying beds. The 
relations seem to suggest that during some disturbance the plicated bed, 
softer than those adjoining, bore the brunt of pressure and became flexed 
while the adjoining beds merely moved in mass. 
The presence of waterworn pebbles of coal and shale in the shales and 
in the sandstone of the Gres Noirs has been taken, very properly, as evidence 
that, by the time that the Grande Couche was complete, a by no means incon¬ 
siderable part of the original lake had been so far filled as to be exposed to 
